I have no idea what chunk type 0x04 is so I have called it CHUNK_MYSTERY

It contains the hardware revision numbers to which that upgrade can be applied. This is the mechanism which prevents Mk1 upgrades being applied to Mk2 players, and vice versa.

There's a history of revision numbers on riocar.org, but basically production Mk1s were revision 5, empeg Mk2s were revision 7, and Rio Mk2as were revision 9.

Peter