I have the 49xd8099bu Android TV.

Neither Kodi nor YouTube deliver 4K video, although the built-in video player will.  Kodi reports 1080p resolution, as does 'Developer Tools' from Google's Play store.

A 4K test file is available from the below link, for those who might want to test the resolution.  http://forum.kodi.tv/showthread.php?tid=261768

Apps can't deliver 2160p unless they know that the screen is capable of delivering it.  How, then can the default be overrulled?