Most Cheers fans will agree that the series can be divided into the Diane Years and the Rebecca Years. Without arguing which is better (Diane), it's easy to see them as two completely different shows that happen to share some characters. The changes in Season Four aren't quite as dramatic, but nevertheless the show isn't quite the same.
The first episode of Cheers Season Two began mere moments after the events of the Season One finale. Conversely, Season Three began several months after the last scene of the Season Two finale. With Season Four, the writers manage to do both: the cold opening begins a few days after Sam flew to stop the wedding, while the episode proper begins months later. It's a clever trick that works well, because there's a lot of plot to deal with.
Season Three's cliffhanger asked (as Diane might put it) "With whom shall Diane end up - Sam or Frasier?" As it turns out, the answer is "Neither," and Season Four will tackle the fallout of that decision.
Sam comes back to the bar first, demoralized by the events in Italy. Frasier comes next, humiliated by Diane and ready to take it out on Sam. The third member of the triangle is nowhere to be found, and soon Sam goes after her.
There's another big change as a new face shows up at the bar. The show has finally acknowledged the passing of both Nicholas Colasanto and Coach. At the time, some critics complained that the show handled Coach's death poorly. In their defense, the writers were dealing with an impossible situation. Sam's comment that he'd like to think Coach is still there is fitting. Characters will continue to refer to Coach, and "Nicky's" photo of Geronimo has become a permanent fixture on the set.
Filling the void left by Coach -or at least trying to- is Woody Harrelson as Woody Boyd, bright-eyed yokel from Indiana. Woody has his own kind of innocence, but he's no substitute for Coach. Whatever his good or bad qualities (and Woody does get annoying as the series goes on) his reactions lack the sweet weirdness Colasanto brought to Coach.
Even with all these changes, this is actually a pretty good episode. Frasier's inept "showdown" with Sam is very funny, and the scenes with Diane at the convent show they can still do goofy and smart at the same time. Having said all that, the show will never quite be the same again.