I have a similar set-up where I had 6000's as rears but swapped them for 4000's on stands. The 4000's are an improvement. Maybe to do with them being the correct head height, whereas the 6000's were firing into the side of the sofa. Generally I think the 4000's sound better than the 6000's. I guess it depends on room layout but for the money I would say 4000's would do the job satisfactorily

Get the 4000s. They'll go well with your BL8000s. Mount them on stands or up on the wall behind your listening position.

I haven't heard the new ones with ICE and don't know of any incompatibility with BL8000s, but if that's an issue then go hunting for a pair of used ones. These speakers will outlast me and used ones will do an excellent job. 

I'm gonna vote BeoLab 4000s. If you use BeoLab 6000s, they often end up firing straight into the back of a sofa, or other furniture. BeoLab 4000s can be mounted at whatever height you want.

Bear in mind that the BeoLab 6000s can be wall mounted now though, if you want to go for them instead, although as others have said, the BeoLab 4000s are kinda slightly better.

Hi,

Definatly go for 4000's, had mine with the same 8000's front  for three years and its a perfect sound!!!!!!!!!!

Ok, one vote from the 6000 camp (looks like I'm in the wilderness compared to the 4000 support 🙂) but I have my 6000's in line with the back of the sofa so they are about 100 degrees around from my seating position and much more involving than the wall mounted Mission 77DS they replaced. These are coupled with BL9 mains and BL7-4 centre, now when does the full-HD BV4-50 hit the dealers...