okay, testing showed that Kodi from the Play Store clearly won't play any 4k files with hardware acceleration
I've copied together some various 4k files (youtube mp4, big buck bunny x.264 2160p@60 mp4, Divx HEVC 24fps mkv showcase Tears of Steel) and put them on a USB stick (why the heck can't this Sony video player app support LAN/SMB?!)
the youtube download out of some reason unknown did not play, BBB looked AMAZING, and Tears of Steel played very well and smooth, even including subtitles. with this stuff you really have to go centimeters in front of the display to appreciate the increased resolution. the details and colors are simply stunning with compression artifacts nowhere to be seen. I really do love this TV set...
so concluding, the Mediatek SoC clearly harnesses lots of video decoding power, it's up to future builds of Kodi to unlock it
on a related note, I completely switched off the HDMI-CEC (BraviaSync) including ARC and went back to optical digital out into my receiver. the whole HDMI-CEC is still very unstable and brought some sudden reboots of the TV set, particularly after waking up from standby. plus, my trusty old Onkyo swallows up jiggawatts while in ARC standby, and I wanted to properly use my Harmony Touch. what's not gone is a slight audio delay with digital out (TV from tuner and all apps), barely noticeable but I do hope Sony fix this soon.