As far as I know the Beolink Converter (the small plastic box - not the amp.) will only connect a Beomaster or a Beovision to a ML-network.
So to connect a BG4500 to the BS9000 you would need a master in between. However I am a not sure that you can connect to Master units e.g. a BS9000 to a BM4500, this depends on one of them being able to be set into slave mode. Some one else can clarify this.
Living room: BV7-40 mkIV + V8000, BL5, BL3, BM1 and BS9000. Bedroom: MX3000 and BL4500 on MCL2-AV. Around: PentaIII, CX100 and MCX35 on ML/MCL + MCL2-A, BeoPort and BL4 on ML, BS3300 + M75 as stand alone, BC6000 + BC600 and BT1100, LC1, LC2, Beo4, Beo5 and BL1000, BS2 and A8, EarSet2, Apron, Coffee mugs, Enamel Bagdes, Bath towel, Keyring, Books, Lots of miniature and the Bottle opener. Office: BC2300 + BL2500 and BS3. Summer house: BS Century.
Addicted? Oh no.... ;)