I love it when people frame it like this; now that they own the monitor the choice is 4k 120Hz or GTF.
Check my signature, I game at high refresh 4k. When I can't get the FPS then I start to move the slider, and if that doesn't work then I drop the resolution to 1440. If it's a 3rd person game that struggles on PC then I'll play it on PS5.
Whilst I'm traditionally a serial upgrader, I certainly won't be dropping £2k+, there's tons of options to exhaust prior to an extreme purchase like that - you know, like all the things you've been doing since before you read the reviews on the 4090.