If you want the PS4 version for Free you will no longer be able to log into the game using the PS3. However.. if you choose to buy the PS4 version you could in effect have both games signed in on the same account giving you a rare option to create MULES by sending yourself Freind Request to yourself.
You cannot do this; it can't currently be done with a PS3 and PC, and thus you won't be able to do it with PS3 and PS4, or PS4 and PC.

but wow, $5 a month to play *any* ps4 game online? fark that, I'd be really suprised if that didn't push more people to join the PC master race.