They neither have 6BG for the gpu, and yet this game is asking for 6GB.
(I'm not saying: you *will need* 6GB or 8GB. I'm saying that before having a consistent idea on what the requirements for "next gen" games are going to be, upgrading your video card can result disappointing).
We don't know for sure if the PS4 matches PC's ultra textures but I think this is unlikely. You don't "need" 6GB.
Maximum settings are completely independant of consoles.