You use the BL4000 as rear speakers I assume.
1, select left/right speaker on rear side of your BL4000 (not line!)
2, you have used the rear speaker output sockets of BeoSystem 3 of your BV7-55
3, you select BL4000 in the DSS setup of your BV7-55
select speaker modes 1-5:
Mode 1: Stereo centre speakers (subwoofer muted)
Mode 2: Stereo in L/R speakers (subwoofer is active)
Mode 3: Dolby 3 stereo
Mode 4: Dual stereo, stereo in L/R front and rear speakers (subwoofer is active)
Mode 5: Dolby digital, Dolby Pro-logic, DTS, Dolby 7.1
That`s it!
BeoVision 8-40, ATV2, BeoSound 4, Front BeoLab 9, Rear Beolab 4000 MKII, Beo 4