I saw multiple posts from people with sudden 4k scaling issues while trying to solve mine so if they find this on a web search perhaps this will solve their problem.
I will try not to editorialize about how I could not duplicate this with other programs that scale to 4k.
I use a 4k monitor and a wider gamut lower resolution monitor in Windows 10.
I normally boot to the 4k monitor or have both turned on at boot time.
Under those circumstances PS CC drop down menus scale properly on both monitors--it only took years for Adobe to get that far with 4k scaling in Windows.
I turned off the 4k monitor while re-calibrating the lower resolution monitor as colors were not matching after an equipment move.
When I turned the 4k monitor back on to resume work, color issues resolved, PS CC drop down menus would not scale no matter what I tried--CS6 redux. Note that PS had not been opened on the 2k monitor, only the calibration program.
As a reboot fixes many problems--voila--things are back to normal booting to the 4k monitor and then turning on the 2k--I have post-its on the monitors now to remind me to avoid this odd behavior in the future.