I am still not convinced season four "failed," but this article is the first to even come close to explaining why it might have that makes any sense to me:
http://www.scriptmag.com/features/craft-features/tv-writing-whats-wrong-with-arrested-development
That article does make a lot of sense, and I agree with all of it. The new season is still good, but it is
different, not just because time has gone by or the plots are new or the actors have aged, but because it feels like it has a different purpose.
I think that when it comes to dramas, stepping outside of predictability and allowing the characters to grow and change is more acceptable, but when it comes to comedy that just isn’t as satisfying. I don’t want to see Michael’s internal moral struggles. Or Gob’s, or George Michael’s, or Buster’s, or Lindsay’s. It’s a real turn off actually, for a comedy. I want to see their outward moral struggles with the rest of their family. And when the rest of the cast can’t be there, or are only there in bits and pieces, we don’t get that. I think the show really only truly succeeds as an ensemble.
The new season is like an entirely different show. It seems that without his family around him, Michael has devolved into his own selfishness and moral bankruptcy. This is the complete opposite of who we were originally led to believe this character was. From the first three seasons, there was always the sense that Michael would be a better person living a happier life if only he could get away from his family, but that he stuck around out of familial loyalty. I really believe that was at the heart of the show – you felt bad for this guy who would never be able to get away, no matter how many times he tried to escape (much like the Gilligan’s Island analogy)! He was just too good of a guy to leave his family hanging, even though they were self-absorbed pricks.
They “changed the rules” with this new season, and with it the premise. He did leave his family, they all left each other, and now we find out Michael is a selfish prick just like the rest of them when his family is not around? Well that’s depressing. It’s certainly not funny. And it’s not the original premise. Before, when Michael made selfish choices it was because being around his family had a bad influence on him, not because he was an inherently selfish person. After watching this new season I kind of hate them all.

Sitcom characters should not undergo such drastic changes. The situations around them should change but the characters themselves just being who they have always been is what makes the show satisfying!