Which character replaced Trapper John as Hawkeye's fellow surgeon in season 4?

B. J. Hunnicutt replaced Trapper John as Hawkeye’s fellow surgeon in season 4 of M*A*S*H.

Played by Mike Farrell, Captain B. J. Hunnicutt arrived when Wayne Rogers departed the series after season 3. Hunnicutt joined Alan Alda’s Hawkeye Pierce in the operating room and in the Swamp, the tent they shared with Major Frank Burns and later Major Charles Winchester. His friendship with Hawkeye became one of the show’s central relationships.

The transition was part of a larger season-4 reshuffle. Harry Morgan joined as Colonel Sherman Potter, replacing Henry Blake as commanding officer, while Hunnicutt replaced Trapper as the camp’s other principal surgeon. Unlike Trapper, who was characterized as Hawkeye’s hard-partying partner, B. J. was portrayed as a devoted husband and father, though he shared Hawkeye’s irreverent humor and opposition to the war.

Charles Winchester is another surgeon, but he did not arrive until season 6, replacing Frank Burns. Sidney Freedman was the camp psychiatrist, and Sherman Potter was the commanding officer and a surgeon—not Hawkeye’s replacement partner.