rmilewsk, is right. The only time your speakers are at 4, 6 or 8ohm is when there just sitting there not playing at all. When in use they will drop and rise in impedance according to what is being played. Most conservative home receivers and seperate amps are stable down to AT LEAST 4ohm. I had an older pair of Paradigm 11SE's rated at 4ohm. Ran those puppies on any and everything imaginable and never had a problem with an amp overheating. I think you will be fine.

Your biggest worry is getting a proper spl balance between all the speakers. And even that's not all too difficult.