Un-plug the BeoSound 9000 or make sure that whatever happens, ONLY the BeoMaster 6500 is in range of the Beo4 remote (or else you are in danger of messing up settings on your BeoSound 9000 etc...).
Push Standby + List at the same time on the Beo4.
Using the up + down arrows, cycle until you reach A.Opt.
Press Go.
Press 2 (assuming the amplifier isn't set up in any special configuration etc...).
The light on the amplifier should flash and the screen should display similar to º2.
The amplifier *should* now respond to remote control signals (assuming my theory is correct - that it was in the wrong 'Option Format'.