Thanks for asking! Below is a list of a few things I really liked about the fourth season without spoilers for those who have not seen it and may be reading this:
1) Although I enjoyed seasons one through three, the story line for season four was a little more believable. I'm not saying it was completely realistic, but it reflected modern anti-terrorism - or at least what I associate with modern anti-terrorism. It reminded me of Zero Dark Thirty (a movie that I really love), which is obviously different in that Homeland is completely fictional, but it had that kind of feel.
2) The production quality was better. The set locations were more interesting to me. Most of the first few seasons were shot in Charlotte, NC. The fourth season was shot in Cape Town, South Africa which really added to the believability and production quality. Interestingly, the fifth season is being filmed in Germany.
3) Character development. The characters that I like were very integral to the story line in season 4 and some of the things I didn't like about them were downplayed. That's about as generic as I can say it.
4) Cliff-hangers. A lot of dramas develop cliff-hangers that are completely contrived and they come across that way to the viewer. The cliff-hangers in season four were much less deliberate and, for me, very effective.
I wasn't crazy about the last episode of season four. I'll absolutely admit that. It was slow at times and I'm not sure how they're going to tie up the loose ends - especially given the next season is being filmed in Germany. Still, I'll give them a pass given the previous episodes were high quality. My theory on the last episode is that it was written to be somber intentionally in memory of James Rebhorn, who actually died during that season.
I'm interested to know What you didn't like about it...