All you'll need to do is buy the PC versions of the game. Consider buying the collectors editions as well as you get some extras that will automatically apply to any of your characters (including new ones you might make in the future.)
DO NOT BUY ON STEAM. Trust me on that. Just buy directly through the Mogstation.
When it comes to updates, the launcher will show Update instead of Play and you'll have to click that to start the updates.
When it comes to picking a PC, be sure you meet all of the recommended specs and ask for help figuring that out if you need to when you go to buy one. The last thing you want is for the PC to either not run FF properly or limit you from doing anything else on it while you're playing FF.
As far as your login, when I swapped to PC from Playstation all I did was purchase the PC versions I was interested in and log in using the login I was using on the PS4 launcher (not your PSN login, the Mogstation login). My characters were still there with the only things missing being, I believe, macros and hotbar set ups. You'll have to redo those either way.
Reddit thread for more info:
https://www.reddit.com/r/ffxiv/comme...s4_to_pc_help/