All of the computer specs mentioned here WILL be able to play Hoi4 but perhaps not so well at full (not limited) speed. The specs will be similar to EU4 and CK2 since its the same engine, though more provinces. Minimum specs can be very low though because its tick-based (each hour in hoi4), so each tick can then take 20+ seconds if need be on a poor system.
I would recommend 4gb+ though, since your pc will have to run the os and the game, and ram usage will be higher for these games (hoi3+black ice mod used 3+ gb ram on my system). If you DONT have enough ram to run though, you will have MASSIVE performance issues and potential crashes.
CPU specs-wise, anything from the Intel Core series (i3, i5, i7) will run it, though those processors have a BIG difference between the worst and best. Better means faster ticks. If you dont have a good enough pc, multiplayer may not work well or at all. Ive not actually played CK2/EU4 multiplayer, so i dont know how that would work on poor performances.
Graphics card shouldnt affect much, since most performance is just processing all those numbers rather than graphic fidelity, but perhaps it could help somewhat. With all that said, HOI4 has some newer features and those MAY require a dedicated video card but I doubt the game would actually REQUIRE one anyway. It shouldnt.
Input, just need mouse and keyboard as youd expect.
Resolution at 1024x768 minimum is easy to do, but id recommend higher at 1600x900 or greater so you can actually see things on the map with those menus (like for aircraft or production) up at the same time.
Operating system, Windows XP or greater (inc Win10), Linux and Mac.
Storage drive speed can help with loading and saving but otherwise just any HDD or SSD will work, as youd expect.
Really, its just CPU performance and RAM capacity that are potential issues for people