I am in a similar situation, my CPU is 4690K (OC to 4.2Ghz) but I have a better GPU as 1080 Ti. I am running mostly Mid settings in 1080p but some options turned to High. I could get good FPS mostly and playable, there are stuttering here and there but I will just have to live with that for now with my limited CPU, and I believe the game need more time to be optimized.
I don't think upgrading the CPU to 4790K will make a huge difference, since you have not mentioned overclocking it may be that one last option to squeeze some last bit of juice from it without spending anything.
Also one more tip I found on Reddit is to go to Windows settings to change processor scheduling from "Programs" to "Background Services", it may help.
