The simple answer:
Do not buy 4K monitors until Windows 10 fixes all its issues with them, which might be years from now.
Much blame lies with 3rd party developers not caring about how their software looks on HiDPI displays — looking at you, Notepad++ — but Microsoft itself has not decided on how they want to handle these displays in a definitive way.
When Apple tells developers it’s their way or the highway, they are lauded for it. When Microsoft does the same, they are dragged through the mud for it. In this instance I’m willing to give Microsoft the benefit of the doubt.
Hell, not even Google has figured out how to make Chrome look pretty in 4k. That’s how bad the situation is.