Running Ubuntu 18.04, I noticed the problem twice after the kernel was upgraded (standard apt upgrade): a 4K resolution just yields a black screen. It falls back from 3840x2160 to 2560x1440.
After installing Ubunut again, it worked again - until the next kernel update.
Setup: nvidia-driver-390 (proprietary, the one shipped with Ubuntu that seems to be based on version 390.48), NVS 5200M. The video card is quite old, but does 4K at 30Hz. Also Windows worked stable in 4K.
Rather than reinstalling Ubuntu after a kernel update, boot with the older kernel.
From the Grub menu, select "Advanced Options" and from there you can select your older (original) kernel that works:
This image generated by grub-menu.sh that displays your grub menu without rebooting. Your real grub menu would look different during boot.
The original kernel version should still work. Then keep trying out new kernels that Ubuntu installs until the bug is fixed. Then after using the most recent kernel you can remove older kernels using:
sudo apt autoremove
