I managed on HassOs supervised by opening SSH and type:
docker exec -it homeassistant /bin/bash
cd /./usr/local/lib/python3.8/site-packages/broadlink
vi __init__.py
type i to get into editor mode
add this line:
0x6539: (rm4, "RM4C mini", "Broadlink"),
hit esc to leave editor mode
type :w to write changes to file
type :q! to exit vi
You should now be able to add the RM4Mini with id 0x6539 immediately
You only need this file change to add devices.
Note that this file change will not survive a server upgrade (however, any added broadlink device remain available once this line is re-added).