Should I get Tropico Trilogy or Tropico 4?

No , don't buy it. it suckS!

For a newcomer, I say go for Tropico 4 as it is the easiest to learn and get into.

The DLCs aren't really needed, they can be useful but nothing really game-changing. The Junta DLC is probably the most useless outta the bunch.

The base game has a pretty lengthy campaign so I suggest just getting Tropico 4 and play it extensively before deciding if you wanna get the expansion Modern Times (preferably during another sale, xmas is pretty near). Especially since the Modern Times campaign has a significant difficulty spike compared to the base game.

Some "Modern Times" buildings also makes the Plantador DLC and the Junta DLC buildings obsolete since the modern buildings are on par or just plain better than the DLC buildings. You can also "quick build" buildings for a cost in the base game and the expansion, making the Quick-Dry Cement DLC a lot less useful once you have a good economy going.