Note that the original release of HOI3 is almost unplayable. It was a great idea on paper, but it didn't work as intended. Sorry, but in my opinion Tibet should NEVER declare war on Argentina without a reason. You need the 1.2 or 1.3 patches just to bring it up to the point where WWII was likely to happen at all. The Semper Fi expansion fixed several serious game-breakers and added a few new features, without gutting the diplomatic system completely (Cores were still a major diplomatic consideration in SF). To me, that's the minimum requirement to call it "playable", and in my opinion it was a pretty good game, at least when it played out without going off into fantasyland. For the Motherland essentially replaced most of the diplomatic aspects of the game with an "on rails" scripted system that traded one set of problems for another. Their Finest Hour added several more aspects and expanded and corrected several issues with the scripted event chains, but didn't "finish" several of the major conversions that FtM started (wargoals were introduced in FtM, and a few more added in TFH, but there are still glaring omissions). To me, it still feels like it's incomplete. In spite of the serious flaws, it's still the best WWII era "simulation" available, and several mods expand on that. If you're willing to invest the time and effort to learn it, it's a gem, but definitely not polished on all faces.
HOI4 is looking to be far more "user friendly", but less historically plausible. I find a lot of the changes "gamey", designed to appeal more to the RTS game crowd rather than the historical strategy gamer. A number of game mechanics will be similar or at least recognizable, so playing HOI3 will probably give you a better understanding of how to play HOI4, but HOI2 might end up as a better "trainer".