4

In mathematics

• Two-dimensional: infinitely many convex regular polygons.
• Three-dimensional: five convex regular polyhedra.
• Four-dimensional: six convex regular polychora.
• Five-dimensional and every higher-dimensional: three regular convex polytopes.

List of basic calculations

Evolution of the glyph

In religion

Buddhism

• Four Noble Truths – Dukkha, Samudaya, Nirodha, Magga
• Four sights – observations which affected Prince Siddhartha deeply and made him realize the sufferings of all beings, and compelled him to begin his spiritual journey—an old man, a sick man, a dead man, and an ascetic
• Four Great Elements – earth, water, fire, and wind
• Four Heavenly Kings
• Four Foundations of Mindfulness – contemplation of the body, contemplation of feelings, contemplation of mind, contemplation of mental objects
• Four Right Exertions
• Four Bases of Power
• Four jhānas
• Four arūpajhānas
• Four Divine Abidings – loving-kindness, compassion, sympathetic joy, and equanimity
• Four stages of enlightenment – stream-enterer, once-returner, non-returner, and arahant
• Four main pilgrimage sites – Lumbini, Bodh Gaya, Sarnath, and Kusinara

  Judeo-Christian symbolism

• The Tetragrammaton is the four-letter name of God.
• Ezekiel has a vision of four living creatures: a man, a lion, an ox, and an eagle.
• The four Matriarchs of Judaism are Sarah, Rebekah, Leah, and Rachel.
• The Four Species are taken as one of the mitzvot on the Jewish holiday of Sukkot.
• The Four Cups of Wine to drink on the Jewish holiday of Passover.
• The Four Questions to be asked on the Jewish holiday of Passover.
• The Four Sons to be dealt with on the Jewish holiday of Passover.
• The Four Expressions of Redemption to be said on the Jewish holiday of Passover.
• The four Gospels: Matthew, Mark, Luke, and John.
• The Four Horsemen of the Apocalypse ride in the Book of Revelation.
• The Four Holy Cities of Judaism: Jerusalem, Hebron, Safed, and Tiberius

  Hinduism

• There are four Vedas: Rigveda, Samaveda, Yajurveda and Atharvaveda.
• In Puruṣārtha, there are four aims of human life: Dharma, Artha, Kāma, Moksha.
• The four stages of life Brahmacharya, Grihastha, Vanaprastha and Sannyasa.
• The four primary castes or strata of society: Brahmana, Kshatriya, Vaishya and Shudra.
• The swastika symbol is traditionally used in Hindu religions as a sign of good luck and signifies good from all four directions.
• The god Brahma has four faces.
• There are four Yugas: Satya, Dwapara, Treta and Kali

  Islam

• Eid al-Adha lasts for four days, from the 10th to the 14th of Dhul Hijja.
• There are four Rashidun or Rightly Guided Caliphs: Abu Bakr, Umar ibn al-Khattab, Uthman ibn Affan and Ali ibn Abi Talib.
• The Four Arch Angels in Islam are: Jibraeel, Mikaeel, Izraeel, and Israfil
• There are four months in which war is not permitted: Muharram, Rajab, Dhu al-Qi'dah and Dhu al-Hijjah.
• There are four Sunni schools of fiqh: Hanafi, Shafi`i, Maliki and Hanbali.
• There are four major Sunni Imams: Abū Ḥanīfa, Muhammad ibn Idris ash-Shafi`i, Malik ibn Anas and Ahmad ibn Hanbal.
• There are four books in Islam: Torah, Zaboor, Injeel, Quran.
• Waiting for four months is ordained for those who take an oath for abstention from their wives.
• The waiting period of the woman whose husband dies, is four months and ten days.
• When Abraham said: “My Lord, show me how You give life to the dead,” Allah said: “Why! Do you have no faith?” Abraham replied: “Yes, but in order that my heart be at rest.” He said: “Then take four birds, and tame them to yourself, then put a part of them on every hill, and summon them; they will come to you flying.
• The respite of four months was granted to give time to the mushriks in Surah At-Tawba so that they should consider their position carefully and decide whether to make preparation for war or to emigrate from the country or to accept Islam.
• Those who accuse honorable women but do not produce four witnesses, flog them with eighty lashes, and do not admit their testimony ever after. They are indeed transgressors.

  Taoism

• Four Symbols of I Ching

  Other

• In a more general sense, numerous mythological and cosmogonical systems consider Four corners of the world as essentially corresponding to the four points of the compass.
• Four is the sacred number of the Zia, an indigenous tribe located in the U.S. state of New Mexico.
• The Chinese, Vietnamese, the Koreans, and the Japanese are superstitious about the number four because it is a homonym for "death" in their languages.
• In Slavic mythology, the god Svetovid has four heads.

  In politics

• Four Freedoms: four fundamental freedoms that Franklin D. Roosevelt declared ought to be enjoyed by everyone in the world: Freedom of Speech, Freedom of Religion, Freedom from Want, Freedom from Fear.
• Gang of Four: Popular name for four Chinese Communist Party leaders who rose to prominence during China's Cultural Revolution, but were ousted in 1976 following the death of Chairman Mao Zedong. Among the four was Mao's widow, Jiang Qing. Since then, many other political factions headed by four people have been called "Gangs of Four".

  In computing

• Four bits are sometimes called a nibble

  In science

• A tetramer is an oligomer formed out of four sub-units.

  In astronomy

• Four terrestrial planets in the Solar system: Mercury, Venus, Earth, and Mars.
• Four giant gas/ice planets in the Solar system: Jupiter, Saturn, Uranus, and Neptune.
• Four of Jupiter's moons are readily visible from Earth with a hobby telescope.
• Messier object M4, a magnitude 7.5 globular cluster in the constellation Scorpius.
• The Roman numeral IV stands for subgiant in the Yerkes spectral classification scheme.
• The Roman numeral IV stands for the fourth-discovered satellite of a planet or minor planet

  In biology

• Four is the number of nucleobase types in DNA and RNA – adenine, guanine, cytosine, thymine.
• Many chordates have four feet, legs or leglike appendages.
• The mammalian heart consists of four chambers.
• Many mammals use four fingers for movement.
• All insects with wings except flies and mosquitos and some others have four wings.
• Insects of the superorder Endopterygota, also known as Holometabola, such as butterflies, ants, bees, beetles, fleas, flies, moths, and wasps, undergo holometabolism—complete metamorphism in four stages—from embryo, to larva, then pupa, and finally the imago.
• In the common ABO blood group system, there are 4 blood types.
• Humans have four canines and four wisdom teeth.
• The cow's stomach is divided in four digestive compartments: reticulum, rumen, omasum and abomasum.

  In chemistry

• Valency of carbon is four. Also because of its tetrahedral crystal bond structure, diamond is the hardest known naturally occurring material. It is also the valence of silicon, whose compounds form the majority of the mass of the Earth's crust.
• The atomic number of beryllium
• There are four basic states of matter: solid, liquid, gas, and plasma.

  In physics

• Special relativity and general relativity treat nature as four-dimensional: 3D regular space and one-dimensional time are treated together and called spacetime. Also, any event E has a light cone composed of four zones of possible communication and cause and effect.
• There are four fundamental forces.
• In statistical mechanics, the four functions inequality is an inequality for four functions on a finite distributive lattice.

  In logic and philosophy

• The symbolic meanings of the number four are linked to those of the cross and the square. "Almost from prehistoric times, the number four was employed to signify what was solid, what could be touched and felt. Its relationship to the cross made it an outstanding symbol of wholeness and universality, a symbol which drew all to itself". Where lines of latitude and longitude intersect, they divide the earth into four proportions. Throughout the world kings and chieftains have been called "lord of the four suns" or "lord of the four quarters of the earth", which is understood to refer to the extent of their powers both territorially and in terms of total control of their subjects' doings.
• The Square of Opposition, in both its Aristotelian version and its Boolean version, consists of four forms: A, I, E, and O.
• In regard to whether two given propositions can have the same truth value, there are four separate logical possibilities: the propositions are subalterns ; subcontraries ; contraries ; or contradictories.
• Aristotle held that there are basically four causes in nature: the material, the formal, the efficient,and the final.
• The Stoics held with four basic categories, all viewed as bodies : substance in the sense of substrate, primary formless matter; quality, matter's organization to differentiate and individualize something, and coming down to a physical ingredient such as pneuma, breath; somehow holding, as in a posture, state, shape, size, action, and somehow holding toward something, as in relative location, familial relation, and so forth.
• Immanuel Kant expounded a table of judgments involving four three-way alternatives, in regard to Quantity, Quality, Relation, Modality, and, based thereupon, a table of four categories, named by the terms just listed, and each with three subcategories.
• Arthur Schopenhauer's doctoral thesis was On the Fourfold Root of the Principle of Sufficient Reason.
• Franz Brentano held that any major philosophical period has four phases: Creative and rapidly progressing with scientific interest and results; then declining through the remaining phases, practical, increasingly skeptical, and literary, mystical, and scientifically worthless—until philosophy is renewed through a new period's first phase.
• C. S. Peirce, usually a trichotomist, discussed four methods for overcoming troublesome uncertainties and achieving secure beliefs: the method of tenacity, the method of authority, the method of congruity, and the fallibilistic, self-correcting method of science ; and four barriers to inquiry, barriers refused by the fallibilist: assertion of absolute certainty; maintaining that something is absolutely unknowable; maintaining that something is absolutely inexplicable because absolutely basic or ultimate; holding that perfect exactitude is possible, especially such as to quite preclude unusual and anomalous phenomena.
• Paul Weiss built a system involving four modes of being: Actualities, Ideality or Possibility, Existence, and God..
• Karl Popper outlined a tetradic schema to describe the growth of theories and, via generalization, also the emergence of new behaviors and living organisms: problem, tentative theory, error-elimination, and new problem.
• John Boyd made his key concept the decision cycle or OODA loop, consisting of four stages: observation, orientation, decision, and action. Boyd held that his decision cycle has philosophical generality, though for strategists the point remains that, through swift decisions, one can disrupt an opponent's decision cycle.
• Richard McKeon outlined four classes of modes of philosophical inquiry: Modes of Being ; Modes of Thought ; Modes of Fact ; Modes of Simplicity —and, corresponding to them, four classes of philosophical semantics: Principles, Methods, Interpretations, and Selections.
• Jonathan Lowe argues in The Four-Category Ontology, 2006, for four categories: kinds, attributes, objects, and modes.
• Four opposed camps of the morality and nature of evil: moral absolutism, amoralism, moral relativism, and moral universalism.

  In technology

• The resin identification code used in recycling to identify low-density polyethylene.
• Most furniture has four legs – tables, chairs, etc.
• The four color process is used for printing.
• Wide use of rectangles because they have effective form and capability for close adjacency to each other.
• In the Rich Text Format specification, language code 4 is for the Chinese language. Codes for regional variants of Chinese are congruent to.
• Credit card machines have four-twelve function keys.
• On most phones, the 4 key is associated with the letters G, H, and I, but on the BlackBerry cellular phone, it is the key for D and F.
• On many computer keyboards, the "4" key may also be used to type the dollar sign if the shift key is held down.
• It is the number of bits in a nibble, equivalent to half a byte
• In internet slang, "4" can replace the word "for". For example, typing "4u" instead of "for you".
• In Leetspeak, "4" may be used to replace the letter "A".
• The TCP/IP stack consists of four layers.

  In transport

• Many internal combustion engines are called four-stroke engines because they complete one thermodynamic cycle in four distinct steps: Intake, compression, power, and exhaust.
• Most vehicles, including motor vehicles, and particularly cars/automobiles and light commercial vehicles have four road wheels.
• "Quattro", meaning four in the Italian language, is used by Audi as a trademark to indicate that all-wheel drive technologies are used on Audi-branded cars. The word "Quattro" was initially used by Audi in 1980 in its original 4WD coupé, the Audi Quattro. Audi also has a privately held subsidiary company called quattro GmbH.
• List of highways numbered 4

  In sports

• In the Australian Football League, the top level of Australian rules football, each team is allowed 4 "interchanges", who can be freely substituted at any time, subject to a limit on the total number of substitutions.
• In baseball:
• *There are four bases in the game: first base, second base, third base, and home plate; to score a run, an offensive player must complete, in the sequence shown, a circuit of those four bases.
• * When a batter receives four pitches that the umpire declares to be "balls" in a single at-bat, a base on balls, informally known as a "walk", is awarded, with the batter sent to first base.
• *For scoring, number 4 is assigned to the second baseman.
• *Four is the most runs that can be scored on any single at bat, whereby all three baserunners and the batter score.
• In basketball, the number four is used to designate the power forward position, often referred to as "the four spot" or "the four".
• In cricket, a four is a specific type of scoring event, whereby the ball crosses the boundary after touching the ground at least one time, scoring four runs. Taking four wickets in four consecutive balls is typically referred to as a double hat trick.
• In rowing, a four refers to a boat for four rowers, with or without coxswain. In rowing nomenclature, 4− represents a coxless four and 4+ represents a coxed four.
• In rugby league:
• * A try is worth 4 points.
• * One of the two starting centres wears the jersey number 4.
• In rugby union:
• * One of the two starting locks wears the jersey number 4.
• * In the standard bonus points system, a point is awarded in the league standings to a team that scores at least 4 tries in a match, regardless of the match result.

  In other fields

• The phrase "four-letter word" is used to describe many swear words in the English language.
• Four is the only number whose name in English has the same number of letters as its value.
• Four is considered an unlucky number in Chinese, Korean, Vietnamese and Japanese cultures mostly in Eastern Asia because it sounds like the word "death". To avoid complaints from people with tetraphobia, many numbered product lines skip the "four": e.g. Nokia cell phones, Palm PDAs, etc. Some buildings skip floor 4 or replace the number with the letter "F", particularly in heavily Asian areas. See tetraphobia and Numbers in Chinese culture.
• The number of characters in a canonical four-character idiom.
• In the NATO phonetic alphabet, the digit 4 is called "fower".
• In Astrology, Cancer is the 4th astrological sign of the Zodiac.

• In Tarot, The Emperor is the fourth trump or Major Arcana card.
• In Tetris, a game named for the Greek word for 4, every shape in the game is formed of 4 blocks each.
• 4 represents the number of Justices on the Supreme Court of the United States necessary to grant a writ of certiorari.
• Number Four is a character in the book series Lorien Legacies

  In music

• In written music, common time is constructed of four beats per measure and a quarter note receives one beat.
• In popular or modern music, the most common time signature is also founded on four beats, i.e., 4/4 having four quarter note beats.
• The common major scale is built on two sets of four notes, where the first and last notes create an octave interval.
• The interval of a perfect fourth is a foundational element of many genres of music, represented in music theory as the tonic and subdominant relationship. Four is also embodied within the circle of fifths, which reveals the interval of four in more active harmonic contexts.
• The typical number of movements in a symphony.
• The number of completed, numbered symphonies by Johannes Brahms.
• The number of strings on a violin, a viola, a cello, double bass, a cuatro and a ukulele, and the number of string pairs on a mandolin.
• "Four calling birds" is the gift on the fourth day of Christmas in the carol "The Twelve Days of Christmas".

  Albums

• Four, British rock band Bloc Party's fourth full-length album.
• Four, British-Irish boy band One Direction's fourth full-length album.
• 4, American artist Beyoncé's fourth album.
• 4, British–American band Foreigner's fourth studio album.

  Groups of four

• Big Four
• Four basic operations of arithmetic: addition, subtraction, multiplication, division.
• Greek classical elements.
• Four seasons: spring, summer, autumn, winter.
• The Four Seasons
• A leap year generally occurs every four years.
• Four is the minimal number of contemporary continents: Americas, Eurafrasia, Sahul, Antarctica.
• Approximately four weeks to a lunar month. Thus the number four is universally an integral part of primitive sacred calendars.
• Four weeks of Advent.
• Four cardinal directions: north, south, east, west.
• Four Temperaments: sanguine, choleric, melancholic, phlegmatic.
• Four Humors: blood, yellow bile, black bile, phlegm.
• Four Great Ancient Capitals of China.
• Four-corner method.
• Cardinal principles.
• Four cardinal virtues: justice, prudence, temperance, fortitude.
• Four suits of playing cards: hearts, diamonds, clubs, spades.
• Four nations of the United Kingdom: England, Wales, Scotland, Northern Ireland.
• Four provinces of Ireland: Munster, Ulster, Leinster, Connacht.
• Four estates: politics, administration, judiciary, journalism. Especially in the expression "Fourth Estate", which means journalism.
• Four Corners is the only location in the United States where four states come together at a single point: Colorado, Utah, New Mexico, and Arizona.
• Four Evangelists – Matthew, Mark, Luke, and John
• Four Doctors of Western Church – Saint Gregory the Great, Saint Ambrose, Saint Augustine, and Saint Jerome
• Four Doctors of Eastern Church – Saint John Chrysostom, Saint Basil the Great, and Gregory of Nazianzus and Saint Athanasius
• Four Galilean moons of Jupiter – Io, Europa, Ganymede, and Callisto
• The Gang of Four was a Chinese communist political faction.
• The Fantastic Four: Mr. Fantastic, The Invisible Woman, The Human Torch, The Thing.
• The Teenage Mutant Ninja Turtles: Leonardo, Michelangelo, Donatello, Raphael
• The Interesting Four: Seiko, The Human Stapler, Weather Woman, and Mr. Wonderful. Parody superhero team featured on NBC's Saturday Night Live.
• The Beatles were also known as the "Fab Four": John Lennon, Ringo Starr, George Harrison, Paul McCartney.
• Gang of Four is a British post-punk rock band formed in the late 1970s.
• Four rivers in the Garden of Eden : Pishon, Gihon, Hiddekel, and P'rat.
• There are also four years in a single Olympiad. Many major international sports competitions follow this cycle, among them the FIFA World Cup and its women's version, the FIBA World Championships for men and women, and the Rugby World Cup.
• There are four limbs on the human body.
• Four Houses of Hogwarts in the Harry Potter series: Gryffindor, Hufflepuff, Ravenclaw, Slytherin.
• Four known continents of the world in the A Song of Ice and Fire series: Westeros, Essos, Sothoryos, Ulthos.
• Each Grand Prix in Nintendo's Mario Kart series is divided into four cups and each cup is divided into four courses. The Mushroom Cup, Flower Cup, Star Cup, and Special Cup make up the Nitro Grand Prix, while the Shell Cup, Banana Cup, Leaf Cup, and the Lightning Cup make up the Retro Grand Prix.