Fortunately, there are a quite few Windows-oriented machines with very decent specs now; some of them have an explicit Linux option, too.
21-22" 4k monitors are indeed pretty rare; 23-24" 4k are more widespread. I would gladly buy a 15-17" 3k / 4k display, like another laptop screen, but they seem to be absent from the market for some reason, despite the wealth of ready-made LCD panels for them.