You're not gonna get good 4K gaming out of a GTX 1070. Even a GTX 1080 would only get you middling results (assuming you're looking for consistent >60 FPS).
http://www.tomshardware.com/reviews/best-cpus,3986.html suggests the Core i7-7700K probably the best CPU for long-term use. That being said, CPUs aren't terribly important for gaming performance, as a rule of thumb. I'm still rocking the i7-960, an 8 year old CPU, and it's still fairly rare for the CPU to be the bottleneck on 1440p gaming, when paired with an AMD Fury X.
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