My setup is: BL8000 front and BL4 as rear, connected to the Avant DD module. The BL4's are hanging upside down on the wall mounts at about 220 cm above the floorlevel. I think they are good enough in the surround-setup for watching DVD's with 5.1 sound. Disadvantage: the direction of the BL4 is fixed in the wall mount, you cannot direct it to the center of the room.

When used as extra speakers when listening to music (SPEAKER4) you notice the difference in quality, I always take care the the BL8000's produce the loudest sound.

In general: I'm satisfied with it. When money was not an issue I think I'll should use a pair of BL3 for a better balanced use when playing music. 

Having the BL1 as front speakers I would say it largely depends on if you listen to surround music recordings or if it is for movie watching.
I have BL3 as front and BL4 as rear. The BL3 are positioned 6-7 meters in front and the BL4 wall mounted just behind the viewing position and yet the BL4 are turned up by 2 in the sound adjustment menu on the Avant.
But for movie watching they do the job perfect.

/Lars

I would look for BeoLab 4000s as rear speakers - BeoLab 4s would really start to sound pushed when you begin to turn the volume up past 'casual' listening levels, in movies and especially in surround sound music.