SHORT CONCLUSION AT THE END IF YOU ARE TO LAZY TO READ MY OPINION(PS: edited opinion at the bottom):

I'm deeply dissapointed in myself that i have to give this game a thumbs down. I only played HOI3 and HOI4, but that is enough to make a comparison between the 2 HOI games.

In this review im going to uphold that HOI4 still is going to get updated and patched. Also im going to uphold one mod that needs to be used to get the full experience of the second world war. That mod is called Black ice.

A brief explanetion of black ice: Black ice is a mod that started in HOI 3 that is currently also developing in HOI4, is a mod that adds extra flesh to the bone. Black ice increases the amount of content in every area of the game. From research to Production to Diplomacy. In both games this made it increase the popularity of the game.

Now that i talked about Black ice lets move to the game. HOI4 brings indeed more ideas to the table and implements them but at the same time it also removes much more than it gives.
HOI4 has a great production ideas and also the national focus tree is really nice. But those are just 2 Positve parts of HOI4 lets start comparing whats better in HOI3 compared to HOI4. HOI3 has a massive tech tree that puts that of HOI4 to shame,
then the entire command hierachy is gone: let me explain that part for you. The command hierachy in HOI3 is that you have a HQ that starts directing commanders around to fronts(the HQ is worth XXXXXX (its the highest in the structure). Then we go over to the Commanders that are directed by the HQ those are worth XXXXX and direct ARMY COMMAND. The ARMY COMMAND is worth XXXX and they will direct the bulk of the armies lower command structure called Corps they are worth XXX. Corps control the divisions XX. The Division is then divided from 1 to 8 brigades worth X. This structure sounds very complicated but it isnt, its actually very nice and realistic. it just goes from high command all the way down to the soldier. every step you can assign generals who give bonuses. Its a Keen part missing in HOI4 that blows life into the game.
The other espects is that the ENTIRE espionage is gone in HOI4(and everybody knows that's ONE of the most important parts in history), The trade and Diplomacy hasn't gotten any improvements and have just been copy-pasted.

SHORT CONCLUSION:
HOI4 is a game that HAS and should have had potential from launch but sadly lost it with keen features missing that were part in HOI3. HOI4 has made improvements in graphics and decreasing the overall difficulty to manage the game, but HOI3s complexity and design are far better then a simple map change and an easier way to control large armies. I recommend you to go buy HOI3 and get the mod Black ice. Even black ice in HOI4 cannot fill the gaps the devs left.

Edit:

I have owned this game for a year now and I think its time to review my opinion.

I have played it for extensive periods of time and have come to a conlcusion.
Although Paradox has Improved the game to a very greate extend they haven't touched the true realm HOI 3 could give us. In the last few updates and future Development Diary's they have talked and implemented many improvements, and what do you know they are copying all features from HOI3. They Hierarchy structure they left out in the beginning now MIRACULOUSLY makes an appearence again. they have just waited for time to sneakely add features they already have implemented in the Previous games.

This is completely outrageous , this is just copy pasting over and over again, im starting to feel like it got the Call of Duty fever where they just copy and past the same and fanboys buying it again and again. Don't buy this game if you want to try to feel how ww2 is like, try HOI3 for the experience and HOI4 for the scenery.