After seeing all 4 seasons, here are my thoughts:
S1: When this came out, it was a fresh wind in the series world. It was fun, crazy and bloody. 9/10
S2: I had heard from friends that this one was really not as good and that story wise it was inferior to S1. I disagree. I really loved it too. 8.5/10
S3: The disappointment. Felt like a long set-up for S4 and it was lacking in fights, violence and wickedness. Still better than most series though. 7.5/10
S4: I feel sorry for those who quit the show before the final season as it was wonderfully bonkers! Last episode was meh but 9 others were flawless. 9/10