I'm a veteran of the series and it took me about an hour to get to grips with the mechanics. So assuming you haven't really gotten into previous games, it might take an afternoon or an evening of play?

It's definitely simplified compared to Hearts of Iron 3. The biggest simplification is probably the Order of Battle, which has essentially been removed completely aside from Theaters (even they have just been reduced to a UI mechanic). I actually miss it somewhat, but I am one of those weird people who enjoyed playing as China or Japan and spending 3 hours setting everything up before even hitting 'play' ...

The downside to all of this of course, is that as you get better and better at the game it ends up becoming much easier than previous titles ever were. It's much easier to do well in Hearts of Iron 4 than it is in Hearts of Iron 3, simply because in order to do well, you don't need to master nearly as many mechanics.

For example, there is next to no emphasis on 'micro-management' of your troop movements and attacks. I'm fairly certain that most players are letting the AI handle 99% of their troop movements, at least that's the impression I get from watching others play.

TL;DR - Yup, it's much easier but less satisfying as well in the long run. I don't think I will spend hundreds of hours on this game like I did with previous titles. But that's fine, I understand that the intent is to appeal to a wider audience and to do that things need to be less esoteric.