It's very early days, but here's a first build with HDR support for RPi4:
LibreELEC-RPi4.arm-9.80-devel-20210205165343-b642f3c.tar
Huge thanks to popcornmix for implementing the missing bits in the video driver!
Please report back if it works with your files / setup.
Note that this is bleeding edge code and so far I only did a very quick test with it which looked quite fine.
so long,
Hias