With 120Hz sure. But there are out there already 4k monitors with much higher refresh rate and often people who buy 4090 level of GPU also buy expensive new monitors with e.g. 240Hz refresh rate. In such case, in quite a few games 4090 seems to be CPU (Ryzen 7 5800x3D) bottlenecked, judging by quite a few reviewers - but only in these very high refresh rates. And no, most games do NOT use single core anymore, I don't know where you've seen only 1 core being used in new games. The main game logic usually runs on single core, but only as it's very hard to parallelize that (it has to wait for other things to happen before it can move to the next step etc.). However, physics, AI, sound etc. - each runs usually in separate thread and so can use other cores at will.