Hearts of Iron IV is at the essence the ultimate grand strategy game about WW2-you command a whole nation. While it's of course possible to play Afghanistan, it goes without saying that most people play it for a certain conflict between 1939 and 1945. And that conflict is very short if you play Luxembourg.
Hearts of Iron have a an increasingly complex gameplay, and with some clamour from the fans, features were cut down from III to IV (presumably to be added later on as DLCs...) Personally, I'm not sure it's always a ''dumbing down''-one feature of III was a very detailed OOB, in which divisions were assembled in corps who made armies who made armies groups who made theatres...It was a neat when commanding middle sized army, and a core when organizing the Red Army mid-game hundreds of divisions with a commander for each one.
The game currently is very playable (a rarity for a Paradox at release), very stable, if a ressource hog at late game. It's considerably easy and straightfoward as a Paradox game goes. The air war system is by far the best compared to the three first games.
The biggest flaw of the game ATM remains the AI, who is noticably worse than the previous iterations.
That the AI react poorly to invasions, encirclements or forget to guard at all their rear areas (ah, invading Japan with 2 Canadian divisions in 1941...) is business as usual. The biggest problem is linked to a great feature, the division builder.
Simply said, the building blocks of your armies, divisions, can be completely customized. It's awesome for an human player, who can indulge to kan-colle level fantasies about armor, light armor, self propelled guns, tanks destroyers....and it's terrible for the AI, who keep spamming dozens of divisions that would have been outgunnned by Italian division binaria. Not only those divisions fold like wet paper when attacked by Human players, but by late game they completely flood the map, which make even decent computers struggle to handle the action and the said action almost unreadable.
TLDR : fun, but the AI pose no challenge whatsoever. In HOI I, II, III a good player (read : one reading strategy guides and microing could for instance handle Germany as France) In HOIV, German AI struggle to beat French AI, and French Player is easily at Berlin by september 1941...