Lucille, GOB, and Tobias are god-tier characters. Michael is just B-tier at best, always has been. Just wanted to get that outta the way.
Anyway, season 4 did a really good job of bringing back most of the recurring characters, but there were a few who didn't make it back [that I can remember off-hand]:
-Martin Short [Uncle Jack]
-Bob Einstein [Larry the Surrogate]
-That one armed guy
-Franklin
Also, young Lucille and Oscar segments were awful, didn't crack a smile even once.