if you already own a 4690K and are happy with frame rates at reduced settings with a lesser GPU the 1070 should let you get at least the same framerates at 1080 high/ultra....
As it is now, no one really intentionally buys a three-generations old 4c/4t CPU any more, save for perhaps one stuffed into a NAS...; the newest games with Windows are pretty much needing 6-8 threads these days, and 8-16 threads is better yet.
If you need a new MB/CPU and RAM, it would be hard to find any better CPU at a great price than the Ryzen 3600 or 3700..
Dude I seriously doubt that CPU is bottlenecking the 1070. Where did you get the idea that it is? If your CPU at 100% during games? Its normal for the GPU to run at almost 100% but not the CPU but considering I have a GTX 1070 and ran it on an overclocked I7-950 (10 years old!!) I don't see this being true.