First, my short review:
Now, the longer version:
I must say I'm really impressed. It takes some kind of effort to turn what was one of the funniest shows ever created (up there with Family Guy and Scrubs) into this absolute pile of crud. Seriously, I barely laughed ever. Oddly, the new character Oliver (replacing Jeff and the most obvious reason the show jumped the shark) provided the source of most of this season's sparse laughs. But, it was mostly slapstick humour, and far too infrequent.
The lack of laughs is clearly the primary problem, what with it being a sitcom, but there are other problems with this sad finale. Mainly that the characters, well established through three previous seasons, don't seem to behave in character. They all seem to have new personalities. Especially Jane, who seems to suddenly be the most clear-headed character on the show!
Also, one of the things that I really love in comedies, and that is done especially well in Scrubs and Family Guy, is when the character's fantasies are played out. While it's done very well in those two shows, it's done very very badly in season 4 of Coupling. Here's a hint for aspiring sitcom writers: having the characters dressed up in silly costumes is not enough, in and of itself, to make it funny.
And do not get me started on the truly, truly awful, cringeworthy, painful and completely not funny way they attempt to bring back Jeff for the final episode.
Mostly, season 4 was killed by the trap that many of these sorts of sitcoms can fall into (especially in later seasons): it lost the balance between drama and comedy. The earlier seasons of Coupling, like Scrubs, had a very good balance of drama and comedy. Actually, Scrubs wrote the book on balancing drama and comedy - the balance on that show is absolutely perfect. Anyway, Coupling slumped into the black hole of drama, and just stopped being funny.