If you are new to the Hearts of Iron series:

You are going to most likely easily spend fifty hours just learning the basics. The HOI series is one of the most complicated strategy games. Even if you get the mechanics down you can still make lots of mistakes in choices. I would highly suggest playing one of the tiny countries to learn the game.

You need to manage your political power, tech research, trading, production, and your national focus. A few different choices here and there can make a big difference later.

If you played Hearts of Iron III:

There is a lot of changes to the game that makes it so much better.

1. The tech is more simplified and streamlined. You get 3-6 tech research slots for each county.

2. Division designer makes it so much easer to manage building your units. Change the template and all units of that template will upgrade or add the brigades. Great example is playing as Germany when you annex Austria and claim their units you can simply click change their template to one you already have designed.

3. The production is broken down into three categories instead of one lump category like it was. You now have Civilian (which constructs buildings), Military (does production lines), and Naval yards.

4. You sorta no longer really build the units anymore, you build the equipment. You can queue multiple units based ony our templates but they are only going to come out as fast as you make the equipment for them.

For example if you want to build a divsions consisting of light tanks, mechanized infantry, and some artillery then you use 1-15 of your military factories to produce the tanks, the infantry weapons, the halftracks, the artillery, and support equipment as production lines.
Each production line makes X amount per day/week/year. While the limit is 15 factories in a single production line, you are not maxed because you can make a second production line with another 1-15 factories. If you change the production lines you will receive a penalty in production for a while (this basically replaces the old practical production.)
When all the vehicles and equipment are made then the unit begins to train. You can send out the units without training or let them remain training for a better combat level.

5. Resources you have Steel, Rubber, Oil, Tungsten, Chromium, and aluminium. Depending on what production lines and how many factories you are running is what resources and how much you need.

6. Doctrines have changed into trees similar to like they were before, however you can only use one line. For example if you favor submarine warfare you are going to lack in the carrier and battleships capabilities.

7. Industry is built as either consolidated for a production bonus or dispersed to reduce bombing damage.

8. Battle plans are much more friendly for the AI. You can draw your own front lines fall back lines, etc and use the general AI to follow that plan. Of course you can still micomanage everything as well.

9. Combat experience is now used to design templates. You can gain combat experience through combat, exercises, political officers, and national focus choices.

10. Reserves are now based on your production lines. Meaning if you are only producing 10 tanks a day and you are losing 20 tanks a day (you can see where this is going.)

11. Officers are gone. In HOI 3 you could easily outproduce units to your officer production.

12. Political officer choices and national focus choices can greatly affect your research now.

Overall HOI 4 feels exactly like a remake should feel. Lots of good changes that put more choices and replayability into the game. On top of the fact playing the smaller countries are more fun and not so much playing the hopeless little guy.

If you are new to the Hearts of Iron series: You are going to most likely easily spend fifty hours just learning the basics. The HOI series is one of the most complicated strategy games. Even if you get the mechanics down you can still make lots of mistakes in choices. I would highly suggest playing one of the tiny countries to learn the game. You need to manage your political power, tech research, trading, production, and your national focus. A few different choices here and there can make a big difference later. If you played Hearts of Iron III: There is a lot of changes to the game that makes it so much better. 1. The tech is more simplified and streamlined. You get 3-6 tech research slots for each county. 2. Division designer makes it so much easer to manage building your units. Change the template and all units of that template will upgrade or add the brigades. Great example is playing as Germany when you annex Austria and claim their units you can simply click change their template to one you already have designed. 3. The production is broken down into three categories instead of one lump category like it was. You now have Civilian (which constructs buildings), Military (does production lines), and Naval yards. 4. You sorta no longer really build the units anymore, you build the equipment. You can queue multiple units based ony our templates but they are only going to come out as fast as you make the equipment for them. For example if you want to build a divsions consisting of light tanks, mechanized infantry, and some artillery then you use 1-15 of your military factories to produce the tanks, the infantry weapons, the halftracks, the artillery, and support equipment as production lines. Each production line makes X amount per day/week/year. While the limit is 15 factories in a single production line, you are not maxed because you can make a second production line with another 1-15 factories. If you change the production lines you will receive a penalty in production for a while (this basically replaces the old practical production.) When all the vehicles and equipment are made then the unit begins to train. You can send out the units without training or let them remain training for a better combat level. 5. Resources you have Steel, Rubber, Oil, Tungsten, Chromium, and aluminium. Depending on what production lines and how many factories you are running is what resources and how much you need. 6. Doctrines have changed into trees similar to like they were before, however you can only use one line. For example if you favor submarine warfare you are going to lack in the carrier and battleships capabilities. 7. Industry is built as either consolidated for a production bonus or dispersed to reduce bombing damage. 8. Battle plans are much more friendly for the AI. You can draw your own front lines fall back lines, etc and use the general AI to follow that plan. Of course you can still micomanage everything as well. 9. Combat experience is now used to design templates. You can gain combat experience through combat, exercises, political officers, and national focus choices. 10. Reserves are now based on your production lines. Meaning if you are only producing 10 tanks a day and you are losing 20 tanks a day (you can see where this is going.) 11. Officers are gone. In HOI 3 you could easily outproduce units to your officer production. 12. Political officer choices and national focus choices can greatly affect your research now. Overall HOI 4 feels exactly like a remake should feel. Lots of good changes that put more choices and replayability into the game. On top of the fact playing the smaller countries are more fun and not so much playing the hopeless little guy.