Earlier quoted context omitted.
Not Windows' fault, though. It's up to developers to update their apps to handle DPI scaling.
True, and I'm pretty sure the reason's the same for Linux. Doesn't affect my opinion on the usability of the platform, though. For my part, I got my work to do, so I ultimately ended up back on a Mac (yet again, ugh) because, for all Apple's recent problems, none of them bother me as much as things like, "I just flat-out can't use this app that I need without a microscope."
Well, at that point, you're no longer using 4K (even if you use a 4K external monitor, OS X doesn't render at that resolution by default). Incidentally, if you do try to render OS X in 4K, it looks horrible, for the exact same reasons.
In my experience, the 4K experience is actually strongest on Linux, followed by Windows. OS X comes in last.