Season four is probably the weakest in terms of writing, but I do find it better than a lot of other shows still - the Inspector Spacetime episode is extremely underrated for that matter. Plus, its the last to feature the original cast - the Greendale Seven, so it does have that in its favor. And season five Season five is pretty good and compact, handling lots of things with relative effciency, including a surprisingly touching send-off to Pierce and Troy (the former not even showing him!) and the return of John Oliver! And I think season six is pretty good, but on the other hand, its easily the weakest of the Harmon bunch. Its pacing is off, and there wasn't a central arc like there was the other years. And Jeff's "agony" about being left behind is only hinted at just a a few episodes before the actual finale - which was, actually pretty good.
In fact, I wish they could've had a section where they'd have Jeff reminisce in the empty study group room how things used to be, and inserted one of the season one webisodes with the Greendale Seven in them in that place. It'd have been wonderful to see that dynamic, even briefly, shown. But otherwise, I liked it.
Anyway, after that derivative point, I'd have to say that while the newer characters were good, at the end they didn't get to really shine on their own much. I wish instead they'd tried to bring John Oliver back and offer some sense of closure with his character there, and while I know Better Call Saul was happening, I do wish they'd kept Jonathan Banks, as well. But thats just me.
And even with the lacklustre seasons, overall its still the funniest, brightest, boldest comedy show I've seen in the last decade, and the comfiest to enjoy its characters since Friends.