A nice city- or island-building experience. It's full of silly banter and stereotypes that manages to stay lighthearted and fun. Modern Times especially has some zany missions, though it doesn't affect the gameplay much.
It's not a hard game. You can ignore faction and foreign demands completely and still keep everyone happy. Just monitor the happiness levels, and check the economic overview to make sure you have enough jobs, workers, and homes. You'll quickly figure out a strategy that works every mission. Just get an economy going, educate your citizens, and improve the lowest happiness category.
Though there are a lot of buildings, operation settings, and policies to choose from, most can be completely ignored. The majority of options are simply not worth fuzzing over or plain worse than the optimal choice. The only thing coming close to a challenge might be preventing gridlocks, which is eliminated by the subway from Modern Times. So I wouldn't bother with DLC for additional options.
Compared to Tropico 3, it's an incremental refinement. Things have been polished and the story is more entertaining. The graphics are of slightly lower quality but the performance is significantly better.