But you lose access to your PS3 game
Today is the day that Final Fantasy XIV: A Realm Reborn PS4 hits Early Access for some folks out there, which also means that you can transfer over your existing PS3 license to upgrade at no additional cost. To be clear, the PS4 version of A Realm Reborn is not "totally free" -- once you've transferred over your PS3 version, it's wiped from your account. Everyone else who doesn't own it on Sony's last console will need to buy the PS4 version separately.
To start the transfer process you'll need to login to the Mog Station and follow these instructions. As I've stated in my write-up, the PS4 port is a huge upgrade from the PS3, and essentially runs the game like a PC on maximum settings. It also utilizes Vita remote play, as well as mouse and keyboard support, and it's cross-play on the same servers as the PC and PS3.
Final Fantasy XIV: Upgrade From PS3 to PS4 [PlayStation Blog]