it's useless to have 120hz with 60 fps going to it

or sell that high refresh rate monitor and get you a nice 75hz with freesync/gsync and maybe that could free up some budget for you to make the components more doable.

my 10700k + 3060ti played d4 beta at ~80-100 fps @ 1080p but what you have to realize is 4k is literally rendering the same picture with 4x as many pixels (granted with more details) and every frame requires 4x the data that a 1080p monitor would.