"Arrested Development" has a very large cast. Many of the members have moved on to other projects and had to steal time to do the fourth season. The problems with scheduling became so severe that Mitch Hurwitz decided to center each episode around one character. That's very different from the season 1 thru 3. Hurwitz has even said that the episodes can be watched out of order. It's a strange beast.
The purpose of season four is to create a foundation for a feature film release.