If you're 100% set on AMD, you'll pretty much need dual cards to have any respectable frame rate at medium settings on 4k. 2x 290/390/RX480. Dual cards are a poor solution though as they frequently don't work right. If you're not 100% set on AMD I'd go with a single 1070 if you're in the $400 range or wait for GP102 and Vega to come out and grab the real high end. Those will likely finally be enough to push 4k at decent framerates with decent settings on a single GPU. I'd personally wait for GP102 or Vega if I was a 4k gamer. I'm waiting for those as a "3K" gamer personally (3x1080p eyefinity aka 3/4 of 4k). My current 290x2 CF struggles at my resolution so 4k is going to be worse yet. Youd have to turn down settings to keep a good frame rate