Hi!
Some point this problem disappeared! Now I can have 4K@60Hz
nvidia-settings: version 390.77 (buildd@lcy01-amd64-022) Thu Sep 6 07:51:39 UTC 2018
The NVIDIA X Server Settings tool.
This program is used to configure the NVIDIA Linux graphics driver.
For more detail, please see the nvidia-settings(1) man page.
Linux WhiteBeast 4.15.0-36-generic #39-Ubuntu SMP Mon Sep 24 16:19:09 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux