No, HOI4 is not turn based game. This game is real-time with pause and speed mechanics. Which you could help slow or speed up the game when you need.
But there is a specific game function which is called ironman which is connected with time and how you play the game. This feature has the name “ironman” and you could select this mode when you start a new game.
What is ironman mode in HOI4?
You could play endless time but you have just one save game file from the current game. So you can’t save the game before major decisions you could just off the game and start where you end last time.
Why play ironman mode?
Because this mode is the only way to get Steam achievements from Hearts of Iron 4.