Tropico IV is different in subtle ways from Tropico III. If you're looking for a full revamp, this isn't it. Most of the visual content is taken straight from Tropico III, and if you enjoy that game and are looking for a major step up this isn't it.
However, it does have a bit of fun with the faction system - you can now take missions from each faction in order to curry favor from each of them, as well as get some subtle rewards. Additionally there's the concept of a council which can both provide beneifts and detriments to your existing setup. Really, it's a refinement of what was there with Tropico III. If you're hungry for what is effectively a really great expansion pack it might be worth picking up at rock-bottom Steam prices. If you're looking for a complete reinvention of the series, you'll need to wait until Tropico V.