When you register the PS4 copy of the game (which is done on the PS4, through the PlayStation Network, rather than through the Mog Station like PC versions are), it will ask you whether you want to create a new service account with SE or use an existing one. Choose the existing account option and put in the same credentials you'd use to log into your PC copy of the game. It should find your existing character(s) and (with another confirmation from you) tie your PS4 copy to the same account as your PC copy, so you can play the same character(s) from either platform.

You'll only have one subscription fee to cover the account and be able to log into that account from either your PS4 or PC (just not both at the same time).

Though you'll have access to your current character, you'll need to do a bunch of setup again, re-creating all your gear sets, macros, hotbars, UI layout, etc.

And if you're going to be switching from using keyboard and mouse as is more common with PC to using the PS4 controller, then you'll need to get used to the controller's cross-hotbars. (There are some configuration options that can make that a bit easier. Let us know if you want advice on it.) If you don't want to switch control schemes, though, the PS4 version can use keyboard and mouse controls. And regardless of whether you use the controller for movement and actions, I'd advise you to at least connect a keyboard to your PS4 for chat.