That's true. Games like minecraft etc. For 60fps on a 4k monitor, you'd be looking at a 1080 for most get and more than a few really require 1080 sli.
With a 1070 you can expect at ultra settings most games getting 30-60fps, but quite a few will see fps drops during extreme detail frames like explosions etc where physX demands can almost bring the gpu to a halt.
To get best playability on a 1070 @4k, it may take relaxing some settings, you'll probably not need things like AA etc at all.
I'd say a lot depends on your gaming style too. If you are preferably into strategy/mmorpg games, that operate at a slower pace, details are more fun. So 4k will look awesome. If your preference is fast paced first person shooters or games like gta:V,then the higher refresh afforded by a 144Hz screen is much more important or details get lost in the movement and ½ the screen is in a constant blur.