That's really excellent - the thing has been coded and designed to work with 4K, but I didn't expect it to work without issue - very very pleased with this. How is performance?
It appears this is a Unity issue with Linux running fullscreen on some distros (here is the bug tracker.)
I'll send technical support an e-mail, see if they are any further along to fixing it. Chances are the issue will disappear when the game is built with a future version of Unity.