The Season Four set has a major bonus: The Family Ties Vacation movie. This is a feature-length film. There is no laugh track, no audience, and many outdoor scenes. It's quite a change seeing the Keaton's outdoors in England. Oh, did I mention the bulk of the film is set in England?
The rest of the DVD set is typical Family Ties stories about family, relationships and moral dilemmas. They took a new direction this season by giving both Alex and Mallory steady love interests with the recurring characters Ellen and Nick. This provided a rich area for exploring relationships and keeping the viewers invested, and I found myself routing for both relationships. Nick was an especially great addition to the cast as a foil for Steven, Elyse, and especially Alex.
Season 4 was at the midway point of the series run, and has all the great aspects and characters that we remember. If you only buy one season of Family Ties, you couldn't go wrong with Season 4.