According to the showrunner, Terry Farrell (Reggie) was written off the show because the show was under pressure from the network to shake things up and improve ratings, going into the fifth season. Since the character of Reggie had departed in a cliffhanger at the end of the previous season, they decided one easy way of injecting new blood into the series would be to have the character not come back at all, and focus on the new character that replaces her.
First episode that does not feature Terry Farrell (Reggie).