No way 120fps at 4k rez. Just not gonna happen on a Switch 2.
But who really cares? I mean I'm sure by then a lot of people will want 4k resolutions, so I could see 4k docked being a major goal for the Switch 2. 120fps who cares? No absolutely games like Mario Kart, etc do not need 120fps. If undocked all games run at 1080p and 60fps that'd be perfect. If docked some games run at 4k/60fps that'd be cool. Though honestly I'd rather have 60fps and more stuff on screen than 4k resolutions.