Oh, do they really now? I've attempted trying to cross-network play my PS3 version on a sub-account and the PS4 version on my main PSN account, but to no avail whatsoever. So either, the network was being finicky, or... I honestly dunno.
I might actually have to retry testing that out to see if there's any plausibility. I want to say that local play is allowed between the PS4 and PSVita, but that's only based off of explanations from others and no confirmed data of my own. As far as I know, online is only a strict basis to the firmware present, AKA PS3-PS3, PS4-PS4, 360-360, XB1-XB1, ect.
... I will actually try to collect some data on this and let you know ASAP. I am very curious now myself.