I think Prime 4 is a cross-gen game at this point.
If it wasn't Nintendo would have shown at least a small trailer for it already, they've never held a game under wraps this long unless it was canned or it shifted platforms.
As for a launch window? I'd guess Mario 3D platformer and Mario Kart Next are going to be the big system sellers early on, with Prime 4 as a tertiary release. Also 3rd party stuff like Final Fantasy VII Remake port, Elden Ring port, maybe a Call of Duty game, that type of stuff.