When you go to play the on the PS4, you must have a PSN account, and a Square-Enix account. The first time in, you will perform a linkage between those two accounts and THAT LINKAGE IS PERMANENT even if it's done during the Beta.

If you want to be able to use your existing characters during phase two of the PS4 Beta, you need to link your PSN account and your existing SE account. In addition, if you have more than one FFXIV Service account on that SE account, you must be sure to select the correct service account in the Beta. And this must all be done correctly in Beta Phase one.

If you do not remember the ID or password of your old PSN account you used on the PS3, you can just make a new one, and use that for the linkage. You can also use the previous PSN account you used on the PS3 provided you can log in to it from the PS4 (remembering the ID and password).

If you have previously linked your PSN and SE accounts from a PS3, you MUST use that linkage. It can not be changed.