Small little update, cause I got actual 4k movie content running smoothly with NFS over UDP (tcp was already too slow). Unfortunately this performance doesn't translate to the Jelly-Fish test files.
jellyfish-120-mbps-4k-uhd-h264.mkv
The smallest Jellyfish 4k Bitrate Test file works "bearable", in that it has visible stutters but is still watchable.
All other 4k test files show heavy stuttering and buffering and are basically unwatchable.
However, as long as actual movie seem to work nicely I achieved the goal I wanted to achieve, especially considering that it was a really cheap box for only 35€.
Thanks for all of your help guys.