The best you could get is either GTX 970, which is 350$, but beware of the famed 3.5GB issue, or you could go with R9 290X, which is a little over 300$. They can both run many games at Ultra, but it's unknown for how long wil they be relevant. 4K is possible, yes, but don't expect 60FPS with max AA aand all Ultra. Also, the R9 290X can get somewhat hot,so be wary