i think the 17.3 disabled the force 4k option (baka nagkaroon ng issues with forcing 4k display on their menu to non 4k supported display), pero from my experience, once you play a 4k source, the player will automatically play it as 4k. to confirm this, use the "info" screen on your tv kung its displaying 4k or 1080 while youre playing your source.
ang issue ko naman with kodi is the audio passthrough.. iffy sya with atmos.. .. so i was forced to use SPMC instead. it uses the older kodi version, but more stable sa audio passthrough.