Now that the season is over, can finally say that I loved that they had the balls to construct the season completely differently to how they had before, with a mystery at the centre of it. They executed it perfectly to my mind. 4 has a real soft spot for me, its right up there with my favourite seasons of the show.
It will be interesting to see how you go with season 5, which is again different from the previous seasons. Ill leave it at that until you finish that one as well.