pcristi (and for all how's competent): All I could figure out from that linked KODI thread jet (tho' didn't have done reading), that in 16.1 KODI it has an auto resolution switch implemented, which can be activated in the Video Settings menu called
Adjust Refreshrate to match video (according to the developers comments it's also refers for resolution, not just fps). So, if I thick this, the resolution will be set automatic to the source (4K for 4K videos and 1080p for 1080p videos) after I start a playback with KODI's own player, right?!
But is this really working at all with this Android version of KODI? I mean even for correct refresh rate switching we had to install your patch under KODI...

:

So it's still a big confusion for me.
And is there a way to set the KODI's GUI to 4K res as well?! All he gets (he has a 4K TV by the way) in the setting menu in KODI is 1080p, and it's even greyed out, so he cannot change it to anything else.
Thanks for the replies! (Maybe Futeko could "drop in his own coin" too in this thread someday...)