David Yost left the show at the end of the fourth season. He later revealed that his departure was because coworkers harassed him for being gay - he was often called a "faggot" and producers would frequently ask the other actors questions about his sexuality.
Yost played Billy Cranston (the Blue Ranger) and appeared in over 200 episodes of Mighty Morphin' Power Rangers during the show's first four seasons! He was also the only Ranger to appear in every episode of the original series. During the fourth season, Power Rangers: Zeo he no longer appeared as a Ranger - instead, he was merely a technical advisor to the newer Rangers. Yost eventually left the show toward the end of the season.
Yost explains why he left in this interview.