A Number Four first appears posing as a human physician treating Kara Thrace for ballistic trauma in what was supposedly a hospital on Caprica. In reality, he was also performing invasive medical tests on her reproductive organs. After Starbuck begins to suspect the doctor's true nature, she kills him, and escapes from the facility. Her fears are confirmed when a second copy of the model greets her on her way. This copy is then gunned down by resistance fighters who rescue her. At least one other copy is seen on Cylon-occupied Caprica. It is later revealed that a Number Four was the Caprica Buccaneers' team doctor and worked against the resistance by contaminating radiation meds and giving too much morpha. This copy was eliminated after Samuel Anders saw the Four copy who cornered Starbuck outside the Cylon facility
The Fleet
Another Four - also named Simon - was a medic in the Colonial Fleet on Gemenon and later lived on the ship Cybele in Galactica's refugee fleet with his human wife Giana O'Neill and stepdaughter Jemmy. This Simon was reluctant to act against humanity because he loved his wife and stepdaughter. A Brother Cavil attempted to pressure him into blowing up the Cybele. Simon airlocks himself instead. Giana learned the truth about her husband after Starbuck returned to the fleet. Although she was distraught, she reasoned he must have killed himself rather than follow the Cylon plan and thus was heroic in a way. According to a Six, he killed himself out of resurrection range and is thus truly dead.
A Four is present in President Gaius Baltar's office during the Battle of New Caprica. One Four, assigned as part of one Basestar's command crew that witnesses a sister Basestar's plight, determines a decision to rescue the infected Basestar cannot be reached scientifically. A Simon captured from that same infected Basestar near the Lion's Head Nebula is interrogated on Galactica. He reveals information about the ship and the Cylons' motives for abandoning it, in exchange for a cure promised by the humans. However, there is no cure, only a treatment. It was thought this Four was killed when Karl Agathon sabotages the environmental control system, killing the Cylon to stop them from being used as biological weapons.
Number Four's role
Simon returns when a Number Three model talks to Baltar in The Passage, after consulting with Simon about the Cylons' next move towards Earth. Number Fours are nearly always featured as a Doctor or Medic, and has a fascination with the possibility of Humanoid Cylons being able to reproduce. He also speaks calmly and clearly, and seems to use logic to solve his problems and make decisions. When Galactica attacks the Colony in the final episode, a Four shows no concern, believing the odds are in the Cylons' favor and there is no need to worry. Ellen Tigh makes the veiled comment Number Four would want to study Caprica Six and Saul Tigh's baby if given the opportunity. Caprica Six takes this to be a threat against her baby. Ellen's veiled threat is proven to be valid, as this Four does get the chance to operate on Hera when she is later kidnapped by Boomer. This Four is killed by Boomer herself who snaps his neck in order to return Hera to her parents during the attack on the Colony. Several Number Fours are among the boarding party led by John Cavil that storms Galactica during the assault on The Colony. At least one is killed by the CIC crew. After a tenuous truce is declared, one Four oversees the transfer of resurrection data from the Final Five to the Colony Hybrids, but the truce is broken when Galen Tyrol murders Tory Foster. The Four in the CIC is shot dead by Starbuck in the ensuing fight. The remaining Fours aboard the Colony all perish when it falls into the naked singularity it was orbiting and is obliterated.
