I've used multiple monitors, but I haven't used a 4k monitor (but I do have a high DPI display).
About multiple monitors: I really like programming with multiple monitors, but lately I've been doing it less and just switch between workspaces/desktops (on OS X and Linux, at least). Multiple monitors requires me to use my mouse more, and I like to keep my hands on my keyboard as much as possible, which is why I'm using a single display more. Other than that, though, it's really nice to have my browser in one monitor with a billion tabs, and then on my other monitor my IDE.
About high resolution displays: High DPI displays can be a pain in the butt to program on (on Windows, at least). Windows sucks at anything but the default DPI settings, unfortunately. I haven't tried on Linux. OS X does well. If you get a high resolution display, make sure it's not too high of a DPI for your OS, or else it might frustrate more than help.