I suppose it can be done.
You will not be able to hear any sources on the BS4 in the link system, but in the room where the BS4 is, you will have a choise of either local sources (FM and CD) or link-sources from the BS9000.
Christian
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.... ;)