How did MASH replace Wayne rogers?

Between Seasons 3 and 4 of the CBS television series M*A*S*H (1972-83), Wayne Rogers decided not to renew his contract. He did not appear on the Season 4 premiere but was definitely a character as Hawkeye tried to catch up with him to say goodbye. As Hawkeye raced around the airport, he met B.J. Hunnicutt, Trapper John's replacement.