2 evga 780 classifieds in sli will handle any game made maxed out at 4k using fxaa, crysis 3, battelfield 4, assassins creed 4, etc. and hold above 40fps.
if you mean 4k ready as in they can push an image or video to a 4k monitor... i believe all current cards above the gtx 650 ti and amd 7750 can do it at 60hz... a couple under can do it at 30hz. any nvidia gtx7xx or amd r7 r9 card can do it at 60hz.
mostly to play games on 50'' 4K TV once and a while. Maybe to Watch 4K Blu-ray if a 4K Blu-ray reader can be put into a pc. .............
im going to guess your looking at one of these new inexpensive 4k tv's, not a $3500 sony or samsung etc.
these seiki etc cheap ones for $500 are great tv's, they are beautiful, absolutely gorgeous. but keep in mind they are limited to 30hz which is 30fps. for bluray or computer 4k media files, this works fine, but for gaming, your going to be capped at 30fps which is less than ideal.
ive been debating on using a 39" for a static desktop, video, picture viewing monitor used in conjunction with my 27" 1440p monitor as the gaming monitor.