From the gtx980ti review at Hardocp
http://www.hardocp.com/article/2015..._980_ti_video_card_gpu_review/12#.VYIVTOlRFjo
quote:
At 4K neither the 980 Ti or TITAN X is actually fast enough to truly enjoy the latest graphically demanding games at high IQ settings, so the VRAM point becomes moot. The main problem at 4K is just the sheer amount of GPU performance needed push all those pixels.
You will need two GTX 980 Ti cards or two TITAN X cards in SLI to genuinely enjoy newer shooter games at high IQ settings at 4K. This is where the benefits of the 6GB of VRAM over 4GB on the GTX 980 will come in handy. It may though, actually not be enough at 4K, but we will test that when we test SLI.
VRAM
As you can see from our gaming lineup (which is using many new games released this year) we aren't seeing much demand over 4GB just yet. There are some hints that some games might need more; Dying Light for example, and possibly Far Cry 4 and GTA V.
We can at least say this, 4GB of VRAM should be the MINIMUM for running games at 1440p today. If you were able to have 6GB of VRAM or more, you will be ensured that games coming this year and next should run fine, as far as VRAM goes at 1440p.
At 4K though 4GB of VRAM is clearly not enough. At 4K you want at a MINIMUM 6GB. It is possible though that more may actually help as you start increasing the number of video cards in SLI. 6GB might actually not be enough for some games in 4K when SLI is involved, we will see.