Excuse my ignorance but I was wondering why the Axiom M 80's are 4 ohm speakers?. What is the advantage of this? Why wouldn't they make them 6 or 8 ohm like their other speakers?

Probably has to do with the way the multiple drivers are wired. With two of everything the choice would have been (nominal) 4 ohms or (nominal) 16 ohms. Since most good amps produce more juice at 4 than at 16 (solid state amps, that is), the decision was easy: make 'em 4 ohms.

Interesting fact:
They have the same number of drivers but the QS4 is 8 ohm, whereas the QS8 is 6 Ohm.

Then again it's a very slow day so my definition of "interesting" is very liberal

But they're totally different drivers.