I think we will see a mid-gen refresh for the current Switch similar to the New 3DS, dropping the price of the current model.
It probably wont do 4k, but will have better battery life, manage more constitent framerates and higher resolutions across the board and give a little bit more headroom for ports.
It would probably get a few exclusives, but nothing world shattering, again much like the the New3DS.
I don't think Nintendo is dropping the hybrid Idea anytime soon, so I'd expect a proper successor to the Switch to arrive 2023/2024 and retain a similar formfactor.
Better screen and chip, possibly native 4k support, wich by 2024 should be much more widespread. We will see what kind of chip it will have, but mobile chipsets are becoming insanely powerful, so if Nvidia actually develops a custom chip, optimized for the Switch 2 we might see a decent generational leap, even over the mid-gen refresh.
If they retain backwards conpatibility, they can still drop the current Switch model further in price and make it an ultra budget machine.