There is online crossplay between PS3 and PS4 on some titles - Guilty Gear Xrd for example. I don't see why it shouldn't be possible here. The two platforms don't need to have the same or similar architecture, the game just needs to have the same server structure on both consoles and the framerate would need to be the same. Seeing as WipEout HD was already 1080p/60fps on PS3, I don't see a problem here. Still; I wouldn't count on them implementing it, probably not worth it for them