Reset the SMC. Now HiDPI 4k working with my external Acer S277HK. System reports 1980x1080 60 Hz in About this Mac->Overview->System Report. A CMD-4 screenshot reports incorrect solution while dragging the box. But once captured the screenshot is in 4K resolution. Confirmed by looking at resolution in Preview or CMD-0 in preview and comparing on my MacBook Pro 13 inch retina screen. Can also confirm by dragging any window from MBP Retina to external screen. Setup is Thunderbolt cable to an Elgato Thunderbolt Dock. Mini Displayport from dock to ACER S277. Also working using displayport direct to monitor without the dock. Did not work before an SMC reset. Then the screenshots were 1980x1080.