The main score is excluding the last 2 seasons. Judging by the first 3 seasons, this is one of the best sitcoms ever made but various factors led to it not doing so well while it ran and getting cut. It became a lot more popular after it had ended and word of mouth got around about how great it was. Due to popular demand and the show being cut so soon, there were rumors about it being revived for years but unfortunately that didn't happen until Netflix decided to pick it up about 7 years after the last season.

I don't know what happened with season 4, but the end result was a season with some big issues and lacking the clever comedy of the first 3 seasons. Season 5 was released nearly 5 years later that unfortunately was also very disappointing. I suspect a key person or people were not involved in season 4 and 5 (like Larry David was not involved in the last few seasons of Seinfeld, though the drop in quality wasn't as severe) or Netflix people butted in too much thinking they knew better.

Again, the first 3 seasons are excellent and up there with other top sitcoms. Several of those sitcoms also had disappointing seasons towards the end, just most had made it at least 5 seasons before the quality started to decline. Arrested Development most likely would have been similar had Fox not canceled it after season 3.