It's true season 4 was decent. If I saw season 4 of Community not knowing the other seasons exist, I would have reacted "This is kinda funny." Just, it was hard to take because they toned down all the things about the first three seasons that made the show special, and made characters act like cartoon caricatures of themselves, more like a standard three camera sitcom than Community. And they only used Brita as the show's Meg, whereas her funniest moments are when they focus on her directionless political rage.
And Changnesia was the most "Standard sitcom" storyline of them all and never led to anything funny.
I thought the Muppet episode was the worst of the bunch. The claymation episode worked because it was always easy to trace what was happening in the cartoon to what was really happening in real life. In the muppet episode there was no connection between cartoon and reality, and they were doing things in the cartoon you know they'd never do in reality like eat strange berries because a stranger told them to. For the concept to work they would have needed to explain why all of them were acting so far out of character in the puppet scenes.
Like Jeff's line in Repilot: You all came here four years ago as real people, and the school turned you into cartoon characters!" That sums up season four rather well.