I've been wrestling with getting better "HiDPI" resolutions on a Philips display, to no avail. I stumbled across BetterDisplay, et voila. Check it out if you're trying to push external displays with a Mac.
Discussion
Im pushing two with a Mac laptop - will check this out. Good looking out
The part that isn't obvious is found here:
https://github.com/waydabber/BetterDisplay/wiki/Fully-scalable-HiDPI-desktop
It's fiddly, but effective.