In a sign that RISC OS 5.28 is hopefully fast approaching, ROOL have released a new download for RaspberryPi with RC 16. You can download it for free from the ROOL website and it needs an SD card bigger than 2GB.
It says it is works with all Raspberry Pi models and includes Pi 4 Model 4 B on the list.
Obviously, this is not final release and comes with all the usual caveats. It is needs testing by the community. So if you want to help speed up the next stable release, it is there to test now.
Downloads page 
