If a game requires 6gb of VRam from your gpu and you have 4 it can start spilling into Ram to catch up. And if you run out of ram it starts caching from your hd. basically your cpu will try anything it can to draw frames... at least thats how i think it works...(or maybe it's just the way the game runs) usually people say 8gb of ram is standard these days anyway.
8GB used to the standard... which is what I have now... but I think you need alil more than that now.
The thing I find Strange is The Phantom Pain didn't hog as much Memory as ground zeroes did.... infact after I quit GZ it will still running in the background for some Reason. I had to finish it off with the Task Manager.
If there actually was any vukan games..I have one and it seems to run better in opengl for me... 8gb of ram should get you by on most games I think and ram being so expensive rn I'd probably upgrade elsewhere before ram if I was you....btw what kinda specs do you have anyway?