If you didn't do the upgrade before the campaign ended then you just need to buy a PS4 copy of them game, install it and register that to your current Final Fantasy XIV account on MogStation and you should be good to go.

People who did the upgrade on MogStation during the campaign got a free upgrade to the PS4 version, but lost access to their previous PS3 version.

http://na.finalfantasyxiv.com/pr/ps4/ps3_users/