For 4k its better to have GTX 980...
But i dnt knw for 3d modeling because i use my cards for pure gaming only...
Also i heard that amd offers a card that is specialized in 3d modeling. Check out the AMD firepro w9100. 16GB GDDR5 and can actually drive upto 6 4k display monitors. Be sure to check out the AMD Firepro series.
Read this if you want to overclock your graphics card...
And one more tip. Buy the MSI brand. Many people have already mentioned in this site plus other sites that overclocking the msi gtx 980 is easier than the other brand. I, myself also have the msi gtx 970 and i can tell ya its hell alot of easy to overclock than the previous one i owned (Asus gtx 770). Also they have pre overclocked settings ready for you.