The GTX 1070 is only going to get you 30-40FPS at 4K unless you're willing to play at low settings, and even then you'll probably cap out at the 50-60 range in most titles. Your CPU isn't going to hold your back in that situation. The only situation where you might face a CPU bottleneck is if you're running 1080p and you have a 144Hz monitor and want to get a locked 144FPS.