Joe is right.
Four pins is for speaker link, and is a predessor for power link. The two extra pins are for display data.
Three pins is for master control link, with one extra pin is for data and one for power.
The reason that speaker link has two extra pins compared to MCL, is that you have to deliver data to both speakers, whereas the MCL carries all infomation incl. sound in one cable, and thereby only requires one combined set of data leeds.
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.... ;)