Having the same problem as @RowanCE. To be clear, it wasn’t just the GUI that was stretched, but the 4k videos as well.
After some digging around, what worked for me (without a reinstall) was to remove my video calibration settings as shown in this thread. I assume I had fiddled with video calibration some time ago and forgot about it.
Afterwards I set the GUI to 4k and it works, as well as the movies. I checked with a test pattern video (example) and it looks good.
However, I believe that the GUI is still in 1080p, only scaled up to 4k. The GUI fonts and the video / artist posters look blocky. That is a pity, the true 4k interface was a real gem. If there is any chance to bring it back, please do so!
Thanks for CoreELEC, as always.