Your questions have largely been explained here: Kodi v19 Matrix is here. Here's what you need to know - OSMC.
- Pi 4 supports 4K output, but you must manually enable this in config.txt.
- No HLG
- HDR EOTF will be sent, but the colour space won’t be correct. This is a known issue on Pi presently.
- HD audio is hit and miss. We need a new version of libasound which will come with Debian Bullseye in a year or so; but some AVRs are clever enough that it works anyway.