The fall out between Lisa Bonet and Bill Cosby can be traced back to her appearence in the movie "Angel Heart". It was a dark twisted movie and she has a scene in it where she has sex with the devil. Mr. Cosby was none to pleased with that. They saw it as an opportunity to create a spin-off. So they shipped the Denise character off to Hillman College in the show "A Different World". However, the Denise character didn't resonate with the fans as much as Dwayne Wayne and Whitley. Denise was soon written out of the show, saying that she dropped out. That was when they brought her back to "Cosby" with her army husband and step-daughter Olivia (Raven Simone) So the reason she doesn't appear at all in the fourth season credits is because the character was actually on the show "A Different World" that year. The reason the character become less important was because Mr. Cosby disapproved of some of the career choices Ms. Bonet made.