There is a lot of software out there that doesn’t “yet” work on Buster, unfortunately. You usually don’t find out until you try to install and or run it.
Just to double check, the 4B booted up with the card from the 3B?
But the fan shim didn’t turn on off like it should.
There is another thread here on getting the Fan Shim Running in Kodi, you’d have to do a search to find it. I posted in it a while back. I believe the author of that addon posted in it?
EDIT: Found it.