Season 4 brought changes to the show:
The title cards have a design, instead of nothing!
Since Paul Stec replaces Dave Smith as the Art Director, the background design has been incredibly improved. There is actual detail to Dexter's lab, house, and city. The storyboards look great now. No more completely horizontal floors or ugly color balance.
Kat Cressida, who voiced DeeDee in Season 2, returns to the role.
The writing is remarkably tightened. No more padding or dragging. The episodes are silly and wacky again, and at no point does the show get boring now.
The episode lengths are now 7-minutes each, because the other technique didn't work. (Instead of 9/3/9 it is 7/7/7)
Season 4 looked like it did much better than Season 3.