Just finished watching season 4 (actually watched the whole series in the last month) which in my opinion, is the best season so far. it felt like a lot more happened then the earlier seasons.
I'd rate them 4, 1, 2, 3.
First season is second because it is when all the characters are introduced and the show is still a new concept. Second season is pretty close, I really enjoyed the whole Doakes thing. I felt the third season was lacking a bit, can't really explain it, just didn't feel as significant as the other seasons.
Which one so far is your favorite?