I really enjoyed Seasoon 4, and I wish that the rest of the series had been more like it. Some of it could have been done in earlier seasons, although maybe not in just one season.
I loved the revelation that Romulans had infiltrated Vulcan society, and had usurped control of the Vulcan High Command, and were the ones who were trying to hold back Earth's technological development (and reinforcing the xenophoia responsible for the birth of such groups as Terra Prime). I would rather have seen this as a major arc of the series, rather than the Temporal Cold War, leading up to the Syrannite Revolution. This could have been the start of the Romulan War, as well, as the Romulans take more direct action to stop the Humans who had disrupted a centuries-long plan.