The higher the resolution the less the cpu matters. Even constant 60 fps is still impossible for the best graphics cards at high settings at 4k, so at that resolution a i5-8400 or an amd would also be enough. For maximizing fps in csgo, you'd want an unlocked intel of course.
Also, the more threads a game utilizes, the harder it becomes to further parallelize work. More threads mean that stuff are calculated on multiple streams that are independent of each other. But, in games, many things depend on previous calculations as opposed to, say, rendering, so at some point it becomes impossible to increase performance by upping thread count.