Gtx1070 is good for 1440p at 60Hz. 2160p will be ok, but don't expect miricles in many of the new AAA titles. For 4k you'll be dropping settings to low/med for most newer games, although Sims and most mmorpgs that don't have the intense detailing will be good for med/high.
For decent fps in 4k at high settings basically requires at least a 1080 and to get ultra in many, a 1080ti. At 4k not even 1080ti sli gets ultra in everything.