Tropico 4 primarily offers technological upgrades, but is still quite fun and addictive. Get the expansion!
Tropico 4 is almost identical to Tropico 3 (and vicariously to previous games in the series) in terms of its mechanics, which are pretty straight-forward: build resources, housing and services, try to stay within the confines of your limited budget, while keeping the various factions of your population under control or risk an uprising, coup-detat or even invasion by a super-power. There is a wide variety of buildings to construct, which will interact with each other to form your economy.
Controls are simple, tooltips are terse but quite sufficient, and it's really a matter of learning to time your construction so that you spend your money on exactly what is needed at any given point during the mission. Simple in theory, difficult in practice, since the mission-specific tasks and random disasters you'll encounter often require you to modify your strategy and adapt quickly to unexpected situations - and that's really where the fun is. Of course, the game's sandbox mode also allows you to get a more open-ended experience, to see how far you can develop your little island. A Multi-player mode is also included, as well as various features to share your accomplishments with your friends over Facebook and Twitter.
Despite having no real gameplay changes from its predecessors, Tropico 4 does offer several advantages. For starters, the technology has improved, and with it the visuals: your island will be significantly prettier than before. Furthermore, it seems like the process of customizing your dictator (with background and skills, giving various advantages and distadvantages) has been expanded, so you can tailor your skills to the missions you are about to undertake. Over time, you can unlock higher advantages to help you with the particularly difficult missions - though personally I stuck with just one set of skills and did just fine.
Finally, I should give kudos to the characterization added in this game. All of the main characters (friendly or otherwise) appearing in the game are fleshed out with voices and caricature portraits which help give the game a very entertaining feel. The story of the main campaign is also quite amusing to follow. My only gripe is with the Presidente's own voice actor, who is sadly under-par (and that's a problem because he obviously has a lot of text in this game), but the other characters make up for this. The random radio broadcasts are often hilarious, with your personal advisors and foreign dignitaries setting an amusing tone to even the worst of disasters.
Unfortunately one aspect that hasn't been improved is traffic - which will quickly become a problem in any growing island - and as far as I can tell there is no way to improve traffic by any noticeable degree. It's a shame that Calypso have not touched on this problem, which was prevalent in each and every one of the previous games. That's really the greatest shame here.
If you enjoy Tropico 4, I would very much recommend that you get the Modern Times expansion pack, which adds a great number of new structures that (over the course of a mission) will gradually replace existing structures with improved versions. The new "timeline" feature allows you to plan ahead for commodity price changes, new buildings, and therefore the changing conditions and situations. However, I should note that the new 12-mission campaign added in Modern Times is significantly harder than the base campaign.
I'm hopeful than Tropico 5, if it gets made, will finally address traffic problems and keep mixing up the gameplay mechanics, since Tropico 4 seems to have already maximized the potential of those mechanics. Modern Times shows how the envelope can be pushed further, so I hope this will be capitalized upon in the next game in the series.