I'm happy not to have read the books... though it does make me wonder...
Anyway, iirc, they are condensing books 4 and 5 into one single season, and they have always suggested the tv series might deviate a little from where the books go.
As long as Tyrion doesn't go anti-Jamie (ie., go from a lovable character to an evil one) I'll be ok.
Also- I loved season 1; and I really liked seasons 2 and 3. But this past season was just out-control-good. Easily the best so far, imo.