After a lot of trial and error, I have managed to find a Linux distribution that works well on my Rockpi4b. It's running Armbian Focal which is based on Ubuntu.
It's set up so I can access and control it using VNC over WiFi which the RockPi4 automatically starts up on boot.
I have managed to compile the stable release of KStars/Ekops from source and they both seem to work OK. I can connect to the simulators in EKOS.
The only issue I have left is that KStars fails when trying to load the extra data in the start-up script and EKOS won't load any of the index files for astrometry.
I have a feeling it's something simple but I have run out of ideas. Anyone got any suggestions.