I multiboxed at 4k with the GT80 sli 980mX2 and it worked very well. I wasn't able to play with everything at max, I turned off AA, SSAO and bloom and water effects and all that jazz.

I don't know if they still do, but the GPU came with 8GB of VRAM each, so that helped a ton with texture streaming and post effects.

I also did it with all 5 accounts on my 4k monitor, and also with 1 client on the 4k screen and the other 4 on a 1080p monitor, but using the dual monitors caused some problems with the cross monitor swapping.