Tropico 4, worth it ? Never played a Tropico before.
Tropico 4 is fun. As you may know they're a series of small city building games where you take the role of "El Presidente" of a banana republic. Fun lies in building your city, abusing your power to fill your swiss bank account, suppressing or otherwise denying your citizens things you find disruptive as well as meeting the wishes of the greater powers in the world around you (US and USSR mostly, though 4 adds Middle East, China and the UN).
In the first Tropico you had the difficulty of trying to balance the happiness of multiple groups that were made up of your citizens. Groups like the Intellectual faction who dislike military buildings and the Militant faction who wish for a well supplied military would make pleasing one be at the cost of the other.
This still exists in the 4th game, though it's not as harsh and it's generally easier to please everyone.
That being said, I can certainly recommend it as it's still a really well made game, and since you haven't got Tropico 3 I can definitely say you should get Tropico 4 (4 is kind of a large expansion to 3) if you enjoy city building and management games. Once you get used to it you might want to up the difficulty a bit to make it harder to please everyone.