4k works fine on my Odroid C2. Perhaps your 4k is h265? If so, try executing this on Kodi startup:
echo 3 | tee /sys/module/amvdec_h265/parameters/double_write_mode
4k works fine on my Odroid C2. Perhaps your 4k is h265? If so, try executing this on Kodi startup:
echo 3 | tee /sys/module/amvdec_h265/parameters/double_write_mode