I've played this game for 60 hours this week. I've got almost too much to say about this game and all things considered this review is waay too long. Unfortunately, I've spent so much time playing this game over the past week that I've barely had time to properly express my thoughts. So let's just vomit some english and see what happens.
TL:DR Version:
https://www.youtube.com/watch?v=r9YCij8CoYk
First and foremost, this is a divisive opinion but I did not like the logistic system of hearts of iron 3 at all. Having to connect brigades and HQs and all that nonsense into a working force was a bit too complicated in my opinion. It turned the micromanagement up to 11 when it should've beena 9. Hearts of Iron 2 and Darkest Hour still had supply systems and intense micromanagement but it was done on a simplistic interface and didn't require you to worry about dragging around dozens of HQ's everywhere your soldiers marched. if HOI was turn based, I could see a system like that working but in realtime it feels more like a gimmick to encumber the player.

HOI IV replaces this system with a much more interesting if slightly imperfect system. Now, commanders are given a set of Army and Battleplan creation tools. The process is simple, you select a number of divisions, click the create army button, set a frontline and then have access to offensive plans, garrison plans etc etc. These plans can then be executed by the AI so a whole theater can be conquered and controlled all by a simple click of the mouse.
What's better, if the plan goes awry there is a simple "Stop" button that pauses unit attacks and forces the AI to restructure along the newly conquered territory.
Don't think this completely replaces micromanagement because you're still able to control units on an individual basis or in selection groups that don't affect the movements and attacks of the rest of the units in the army.

While the overall idea and execution are pretty fantastic, there are a few hiccups... like the AI sometimes not advancing even when a front is left completely undefended. This is the most common problem and is generally solved by direct player intervention. Sometimes the frontline drawing tool can mess up causing units to stack up on the wrong provinces, again this can be solved by creating a new Army and resetting your frontline. Other issues include clutter. When multiple armies' objectives connect, the screen becomes easily cluttered by offensive line indicators. In some situations it just feels like you're better off moving troops by yourself.

In summation, I love the new battleplan system. It really takes a load off of the player and allows you to just watch your "generals" take control of the war. It especially comes in handy when you are fighting in multiple theaters and at times is an absolute godsend.

Air war has also become a bit more streamlined in Hearts of Iron IV. No longer do you directly control aircraft like your land divisions. Air Wings are given their own panel which shows where planes are, what they are doing, their effectiveness and even keeps track of how many are lost or how many kills you've made. You're probably wondering how this works, well simply put,you assign planes to an air wing which is restricted by type of plane and then you
assign that air wing to an air region and then rebase them to the nearest air base. Once there, you can choose a mission for them depending on their type, so air superiority, air interception, strategic bombing, ground support and so on. Plans will then conduct their missions as long as they are still alive in the area or until all your enemies are dead. You can customize their behavior to a degree by adjusting when they conduct missions and so on.
If your planes are losing the air war in a region, you can even ground them to try and minimize losses until you have the necessary number to fight back.

Hearts of Iron IV also changes up the industrial system in the game. In other games you would use sliders to assign industrial capacity to production, consumer goods etc etc. Now, you have 3 different types of factories; First is Civilian Factories which are used to trade for resources, construct buildings and automatically produce consumer goods depending on your country's economic laws. Second you have Military factories which produces all infantry weapons,
tanks, and airplanes. Finally, you have Naval Dockyards which produce convoys and various naval vessels. While there are more buildings such as the nuclear reactor and synthetic refinery, the primary producers are these three buildings.
Personally, I enjoy this new take on production. It allows you to take direct control of production by swapping factory output on the fly as the war drags on. And now because each unit costs up to 3 different resources there is more importance in effectively managing your economic situation... do you trade off construction speed for more KV-1 production or do you wait and build more synthetic refineries? Personally, love this new system.

On the discussion of production and creation, research has also been changed a bit. Where the Hearts of Iron 3 tech tree decided to create far more options than could ever be researched in one game, HOI 4 has gone back to the Hearts of Iron 2 approach with general research options that unlock new units and technologies the further down you go.
What I love the most about this new research tree is even small countries have a chance to keep up in technology as long as they focus on certain areas. Yes, this means that Mexico can build nuclear weapons by around 1945 if you tech right and have enough research slots.
Some of you are probably sad and feel like now you can't customize your land armies like you could in HOI 3 because of the simplified tech tree, well there is more customization now then there ever was. For example, in the production panel, when you are creating a tank, or an airplane, you can customize the level of gun, engine, reliability, and armor of your tank using Army experience that is gained in your "hopefully" never ending conflicts.
This leads to your tanks having increased utility even if they are a few years behind on the tech tree. It's worth mentioning the national focus system as well as this allows you to customize your playstyle as whatever country you are playing as. For example, if you are playing as the Soviet Union, it gives you bonus towards an anti fascist playstyle or as an anti capitalistic playstyle depending on who you are trying to ally with in the war. National Focus can also give bonuses towards factories, troop recovery rates, research slots and so on so forth.

The last major change I will discuss is the Create Faction feature of the game. In other Hearts of Iron games, you were limited to three different factions, Comintern, Allies and Axis. In HOI IV you can actually create your own faction... or a preset faction already available. For example, Italy can form the Novus Imperium Romanum or join the Axis, Allies, or Comintern. I played a game as Mexcio where I created my own South American Faction called the Imperio de Sud and proceeded to grab as many south american countries as I could.
It's actually a really cool system that makes staging coups all over the world a freaking treat.

Truly, the only flaw or issue I have with this game is that the AI can be strange. It makes a mixture of unrealistically unpredictable and sometimes completely idiotic decisions. Of which I'm sure you will experience. I also feel it should be mentioned that due to some of the AI issues I've experienced, it makes me think that HOIIV is more geared towards multiplayer. Keep in mind that's a good thing for those who get bored with the AI easily. Can't wait to see some additional content and get going with some multiplayer matches.

Highly recommended to newcomers of the IP and veterans alike.