The way it works as far as I recall if it's being done the same way is. You turn your PS3 HW into PS4 HW but you lose access to PS3 HW.

http://na.finalfantasyxiv.com/lodest...06d20c1826d455

To confirm:

"Purchase of the PlayStation®4 version is not required to upgrade. Please be aware that after transferring to the PlayStation®4 you will be unable to play using your current copy of the PlayStation®3 version."