[gamble]

Yes, i do use the mini HDMI Output from the nuc and if i connect it directly to my TV Kodi switches to 4K without problem.
Only thing NOT working is 4K Output when connected trough my Marantz AVR, then the Marantz tells the NUC via EDID
that UHD is NOT an available resolution..

edit.
I am one step further now.

If i activate the scaler in the Marantz and force it to upscale ALL inputs to 4K it works, 4K resolution gets available in the EDID
and passed trough as the marantz does no altering to the image if 4K is input..

So what is the problem now u may ask...only downside now is that ALL stuff get upscaled to 2160p..1080p movies, 720p tv shows etc.
i still can not just pass-trough the original signal

But thats no big deal, for now i even set Kodi GUI to 2160p, thumbs never looked better..thats a night and day difference
Seems like Kodi now also scales everything to 4K if my Marantz is telling the truth in HDMI-INFO Screen...i think i remember someone say that KODI does an awful job at scaling
but while comparing to the marantz and sony scaler i see no major differences...in the few minutes i watched i even have to say that i liked Kodis scaling best (just watched a few minutes of the new Robocop..)