1440p it would work flawlessly with 60FPS at higher detail levels, but at 4K they recommend a 1080 at least to play mid-high graphics at 60FPS. A 1080TI can play very-high graphics at 60FPS.
If you down-scaled from 4K to 1440p on a 4K monitor, it would look fuzzy. So your essentially stuck at gaming on a 4K monitor with low quality graphics at 4K < 60FPS or fuzzy-looking games at 1440p with mid-high graphics > 60FPS.
There is really no win there in my opinion but if someone knows better than I about gaming at 4K with a GTX 1070, I'd like to know.