I tried searching for this info but had no luck. Can Kodi output Ultra HD BluRay resolution? What about 4K? If not, is it a Kodi issue or a hardware issue? I am building a theater room and am curious if I will need a hardware BluRay player to output those resolutions.
It all depends on the hardware you run Kodi onto, really...
But yes, Kodi itself supports 4k
As the Lord God Koying himself says it depends on the Hardware, Ultra HD blu ray is encoded at 3840x2160 with 10 bit color and whatever color space the encoder chooses to encode to within a rec2020 wrapper with a chroma sub sample of 4:2:0, virtually all Ultra HD blu Ray disk releases have been released with 24hz frame rate and HDR metadata also.
If this is what you want to view then you will need a dedicated UHD blu ray player.
Bear in mind it is currently impossible to crack the protection on these disks.
If you just want to be able to play captures of these disks, or other 4K media files Kodi and its many forks are capable, the hardware not so.
At this moment in time if you wanted to play back a 4K hevc 10 bit 24hz file natively with HD audio you would need a PC based system with a HDMI 2.0 specced output.
The best android boxes are all struggling with crippled firmware / chips.
Mr Koying and others are doing their best with their own developments but don't seem to get any help further up the ladder.