If you currently had the PC standard edition (for example) and now bought the PS4 CE you could register that to the SAME account to unlock both platforms for that account, and granting the CE items to all characters on the account (including the one you made on PC) and could then play your old character on both platforms if you wanted.

If, however, you had the PC standard edition and bought the PC collector's edition you would NOT be able to register it to the same account because you already have a PC version registered. In this scenario you would have to buy the collector's edition upgrade on the mogstation to upgrade your account.

Hopefully that clears it up.