Jeez. The sentence, "I booted distro-X on it and trying to deal with the resolution issues from was just a huge headache," is still true in 2018. People were saying things like this about Linux 15 years ago!
I find KDE to be fine with a single 4K display. I think the difficulties come with mixing a standard and a HiDPI screen.
This is a more common use case than some folks might imagine. My laptop screen is HiDPI, but as soon as I plug into a projector or screen sharing device that acts as a second display, things get realllllly weird: Linux/X does not know how to dynamically change screen DPI without logout/login. I work around it by just running my laptop screen at "low" DPI. I don't miss it TBH; I suspect that many people who do notice it have better eyesight than me (possibly an age thing).
Wayland is supposed to help with this to some extent, but it's not yet ready for prime time.
This was a few years ago, maybe things have improved.