Square Enix is calling it a beta, but it does connect to the servers PS3 and PC players are using, so PS4 heroes-to-be can hook up with everyone else already in Eorzea. Existing characters are eligible for the phase, and progress is transferred to the game proper when it launches fully on April 14.
It's probably best to think of it as Super Early Access weekend that's free to all, and get in while you can because it ends on April 7 at 1AM PT/4AM ET/9AM UK time. For the record, the official Early Access is available to anyone who pre-orders the PS4 MMORPG, and that's scheduled to get going on April 11.
Final Fantasy XIV: A Realm Reborn