Your Father Should Know, Pt. 1

Despited 401 and 402 being a two part episode, part 1 was originally produced as a season 3 episode which was held over for season 4 once the show was cancelled by Fox and picked up by The WB months later. Part 2 was produced in season 4 to go along with it which explains why Bret Harrison is a guest star in part 1 but main cast in part 2. This also is a common occurrence in season 3 where he was added as main cast later, and due to it being aired out of order like many comedies, he alternates between guest star and main cast throughout the season as well as the 6 episodes from season 3 that aired in season 4.