Sierra Drops HiDPI Scaling on External Displays
Tim Hardwick (via Mike Rundle):
Several MacRumors readers using 4K displays have described the disappearance of scaled HiDPI resolutions beyond 1920x1080 since updating from OS X El Capitan, leaving their options reduced to 1080p or a non-scaled native resolution of 3840x2160. Monitors with native 1440p resolutions also seem to have had HiDPI scaling options removed.
Owners of larger displays often use Apple's scaled HiDPI resolutions offered in Display preferences to enjoy increased desktop space without reducing the legibility of text.
This command in Terminal and a reboot fixed it for me on Sierra.
See also: Super User.
Update (2016-09-27): Ian Sutherland:
I was having this problem with my 4K display in Sierra. Plugged it in today and it worked perfectly at 2560x1440. No idea why.
