I've decided to make a guide on hotkeys because I often see people who don't use them at all, even in multiplayer, where speed of your micro management is very important. I will also show some other hotkeys that not many people know about, even though they are useful.

• S - Split selected divisions
  One of the most useful features in infantry management is 'split'. It lets you quickly assign orders to a group of divisions, and using a hotkey for this saves you a lot of time.
  
  
  
• Shift+K - Exercise until trained
  It's one of those features that not many people know about. It lets you exercise divisions which experience level is lower than 3, without exercising and spending equipment of divisions that are already at third experience level.
  
  as you can see, only untrained divisions are attritioning
  
  
• Ctrl+B - Toggle Strategic Redeployment
  Makes selected divisions railroad to their destination point. Works even if they are already on their way.
  
  
  
• Ctrl+1-9 to assign, 1-9 to select - Creates a shortcut for selected divisions
  Another unknown to many players hotkey. It lets you quickly select assigned divisions or jump to their location (if you click two times). I use it for tanks or other strong divisions.
  
  
  
• Ctrl+Right click on an attack bubble - Support Attack
  Divisions that are supporting an attack dont move away from their position after they managed to make the enemy retreat. Especially useful when divisions are defending from the other sides and you don't want them to leave their tile after the attack is over.
  
  
  
• Shift + Right click - Creates a waypoint for selected divisions
  A very convenient and commonly used feature that allows you to navigate a path for your divisions.

Quick army management is important as well. It's easy to find shortcuts for battleplan management so I will show only the most important ones.

• Z - Create a front line
  
• Ctrl+Left Click - Assign divisions to a front line
  
• X - Create an offensive line
  
• Alt - Edit Mode
  This one is useful to edit your offensive lines without losing battleplan bonus after your army almost reached the end of it

• Backspace - Return to capital
  Useful when you hear a naval invasion sound and you have to quickly return to your country to defend from the invasion
  
  
  
• Ctrl + 1-9 - Creates a shortcut for a province
  Select a province and then press Ctrl + number. Lets you quickly move between the fronts.
  

• Ctrl+F9 - Toggles the Interface on/off for screenshots
  
  
  
  
• Ctrl + F10 - Creates a political map of current session
  This one is cool, I never knew about it before doing my little research on HOI4 shortcuts. Screenshots are located in the game's folder in documents.
  
  Path to it is: C:\Users\YourUsename\Documents\Paradox Interactive\Hearts of Iron IV\Screenshots
  

Decided to share a tip about navy, because why not.

There is an easy way to merge all your fleets into one. Useful for Japan and the UK.

Select a fleet and then Shift+Right click on other fleets


Assign selected fleets to a reserve fleet


Then press 'G'


That's it