Largely because the second half of that season contains most of my least favorite episodes (including my absolute least favorite, "You Gotta Have Hope"). The first half of Season 4 is pretty good, but from "Valentine's Day" on, I like only 1-2 episodes the rest of that season.
I want to say that's around the time the new writing crew took over.
"hey when you're in the desert you don't complain that the champagne's not Korbel if you get my drift." - RightOnDude