Community can have some really great writing (dips in season 3, but recovers in 4).

Is this an "arrays start at 0" joke? The 4th season is the one where Dan Harmon was booted off and the quality dipped significantly. It can safely be skipped even if it's your first watch.

All you need to know is that they wrote out Pierce partway through the season, Abed gets a gf (but she's only in like 1 or 2 episodes of season 5), and the gang all graduate from their programs and go on to get some kind of job. Everything else is explained fairly quickly in the 5th season.

The writers basically did everything in their power to erase the mess that the 4th season was from existence. The characters refer to it as the school year where there was a gas leak in the whole school and that's why everyone acts weird and doesn't remember anything.