Topic on Talk:Quake 4
Seems that you can't just set 4K resolution with Q4T - game's ignore the setting and launch in whatever resolution you've had selected in it previously.
A simple workaround is to go into Quake4Tweaker.ini and comment out CustomResolutionWidth line - this does the trick and you get 4K working just fine.
I don't have enough sample h/w to check if this applies to other resolutions too. If anyone is willing to test this fix and it will also help with other monitors / resolutions it should probably be included into "Widescreen resolution" section.