Running Linux. I don't have a 4k monitor, but multiple monitors are trivial to set up and automate with XRANDR. In addition, window managers like I3 allow for fully customizable, keyboard-driven navigation between screens and windows. Not saying there are no problems, but once you figure things out the setup is much more extensible than what I was experiencing on Windows previously (former Windows dev here).

Mixed HiDPI with regular resolutions are horrible in Linux, I use it daily and probably tried most DE's and WM's, problem is that it will either look bad on the HiDPI-monitors or the standard ones. The resolution to dot independence in OS X is probably one of the strongest features today, especially paired with quick resolution switchers.

I guess you tried it with Unity then. Monitor support and how it feels strongly depends on the Desktop environment used. I never had a issue with Gnome Shell or even Fluxbox, it is even way less of a pain than the crazy bugs OSX seems to have with multi monitor setups.