I'm going to say you should technically be alright with only 4GB as long as you're okay with turning down some settings, or using DX9. High settings in DX11 are going to eat through VRAM very quickly, and I've already been able to push 7GB in 1440 (with probably ~700MB - 1,000MB of that being added from Chrome). Now, that's probably a bloated number since a lot of games seem to load things into VRAM, and then not flush them out immediately if there is an excess amount of available VRAM.