Number Four (Simon O'Neill)
Simon first appears in the episode "The Farm", posing as a human physician treating Starbuck for a gunshot wound in what was supposedly a resistance hospital on Caprica. In reality this was a Cylon "farm" set up to breed human-Cylon hybrids using captured humans, and he was also performing invasive tests on her reproductive organs. After Starbuck began to suspect Simon's true nature and killed him, she escaped from the facility and her fears were confirmed when a second copy of the model greeted her on her way out. He was revealed to be the 4th model in the episode "Six of One". Fours are consistently the Cylons' medical specialists.
Another Number four appeared in The Plan, posing as a married man in the Colonial Fleet. Cavil at one point asks Simon to leave his life behind and destroy the ship he lives on. He defies this order in order to protect his wife and her child by committing suicide and flying out of an air lock. He was not close enough to a Resurrection Ship and therefore died permanently.
Number Four are played by Rick Worthy.