As I suspected, the Beo4 is not to blame here. Older BeoSound 9000's were designed to check their PowerLink connections when they are powered up, and if there is no B&O speaker attached they go into Audio Option 0 automatically, assuming that a B&O television must be nearby, or that all speakers are on links.
This behavior is normal. B&O offer a plug which you can plug into one of the PL sockets and fool the 900 into staying in Audio Option 1. The part number is 6271061, but I don't have the current pricing at my fingertips right now.
Russ