Works with Wasabi Mango UHD420 (42 inch 4K) !

I tried the plist terminal command, etc. and nothing was working to enable HiDPI modes.

Installed this little app and boom! - You can switch between HiDPI and standard resolutions with ease.

On my UHD420 it actually enabled 14 HiDPI resolutions to pick from! Native resolution for this display is 3840x2160 and it enabled a varying array of HiDPI resolutions ranging from 3200x1800 HiDPI down through 800x450 HiDPI (of course including the standards of 2560x1440, 1920x1080, 1280x720, etc.). It also had the same number (plus some) of standard non-HiDPI resolutions to pick from as well.

For the record this was on El Capitan.

Good stuff!

TheLostBrain about ResolutionTab