[M4tt0]

Did some more tests and found the following:

The 4k TV streams, which activate HDR HLG correctly, are decoded using am-h254 (HW), Deinterlace-Methode: hardware
The 4k Youtube streams, which do not activate HDR correctly, are decoded using am-vp9 (HW), Deinterlace-Methode: hardware
I've also downloaded that same 4k Youtube video. Trying to play it via the filemanager failed, as videplayer tried to software-decode it, which the N2 (of course) cannot handle

FWIW...