You should check with gaming gaf to be sure. I've not got ps+ at the moment, but it worked with my ps3 and Vita so it should for PS3 to PS4.That's what I figured. Just wanted to be sure - even if it's a daft question - as I've never really read up on it before (and never used PS+ before).
I'll definitely need it to play online on PS4 but was wondering if I should just go ahead and purchase the subscription now for my PS3 so it's out the way.