Charles Emerson Winchester III was introduced as a main character in Season 6 of M*A*S*H.
David Ogden Stiers joined the series as Major Charles Emerson Winchester III after Larry Linville’s departure as Frank Burns. Winchester first appeared during the sixth season as the replacement surgeon assigned to the 4077th, creating a new professional and comic foil for Hawkeye Pierce and B.J. Hunnicutt.
Although Winchester initially filled the structural role Frank Burns had occupied, the characters were deliberately different. Winchester was aristocratic, formal, proud, and frequently abrasive, but he was also exceptionally skilled, intelligent, and capable of compassion. His conflicts with Hawkeye were often arguments between two highly competent surgeons rather than simple bullying.
The character’s full name also has a small continuity wrinkle: the “III” suffix was not used consistently at first and was added to the character’s name by Season 8. That detail does not change his introduction point. Seasons 4, 5, and 8 therefore cannot be the answer to when Winchester joined the main cast.