Terrifying wax figures of renowned personalities, such as Attila the Hun and Jack the Ripper, surround the sale of a London museum.

Karen marries Arnold at his funeral and continues to get his money as long as she stays by his coffin. Meanwhile, various oddball relatives after Arnold's wealth are being killed in a creative variety of ways.

KITT's dashboard was redesigned, and he gained some new abilities. Patricia McPherson returned as Bonnie Barstow, and would remain until the end of the series. Michael and Devon had also become close friends as well as associates by this point, trusting one another enough to work and conspire in secret together without notifying KITT or Bonnie ("Knight in Disgrace"). KITT's renegade prototype KARR resurfaced in the episode "K.I.T.T. vs. K.A.R.R". A new voice actor, Paul Frees, replaced KARR's original voice actor, Peter Cullen.

Interest in the show declined, leading to its cancellation by the end of the season. Hasselhoff, knowing that the show was coming to an end, attempted to make a "series finale" and helped conceive the story "The Scent of Roses" with his wife at the time, Catherine Hickland, who played Michael's love interest Stephanie Mason in past episodes. The network instead aired "Voodoo Knight" as the final episode. A new street-smart mechanic, played by Peter Parros, was added to the cast in addition to Bonnie. Of all KITT's features added in the fourth season, the most important of them is "Super Pursuit Mode". KITT could also convert into an open convertible. Both cars were designed and built by customizer George Barris. This season also marks the final appearance of Bonnie Barstow, RC3, and the Knight Industries Two Thousand in its original form. KITT's final appearance in the Knight Rider franchise was in the 1991 made-for-TV movie Knight Rider 2000.

In season two, Patricia McPherson who played Bonnie Barstow, departed the show and was replaced by Rebecca Holden as April Curtis, the new technician for KITT. April was written out after Holden left the show at the end of season two, and McPherson returned for season three until the show ended.