The 1070 should do 40 fps average on current games' ultra settings in 4K. Bringing things down a notch to high settings should possibly net you 50-60 fps, which is in playable ballpark for me.
The 980ti was my go to 4K card for a year or so, and the 1070 is 30% faster than that (on paper at least)
Here are some benches for reference
http://www.tomshardware.com/reviews/nvidia-geforce-gtx-1070-8gb-pascal-performance,4585-3.html