I'm having some issues with my dual displays using this configuration. ArcMap never remembers which monitor it was opened on - it has to be moved every time. Fonts and icons are incredibly tiny. I changed the icons in options, but they don't stick - I have to set them every time. When I open an attribute table and try to drag it the display, I have to dock it for it it stay on a display. And then undock it on the display where I want it. None of these are deal breakers, but they are incredibly annoying. I have not been able to pin this issue down - is it the 4K laptop display (although my external monitor is not 4K - it is 1920x1080). Is is Windows 8.1? Is it ArcMap? If I set font sizes for various menus, palettes, etc those changes effect ALL of my programs, most of which seem to behave fairly well.
If anybody is using a similar setup and has experimented with fixes or work-arounds, I'd love to hear about them.