Peter, the 4310 will drive the M80s to levels beyond what is safe for your hearing. This includes the frequencies where the M80s have a 4 ohm impedance, but note that the M80s are 8 ohm speakers over the majority of their frequency range.
There is no bi-amping capability in the 4310, despite the language in the manual. There is only one amplifier, with seven output channels to distribute the limited amount of power of which the power supply section is capable. So, there are no "amps to waste" if you're not using back surrounds or zone 2, and no increase of power to the mains is available by routing it through two sets of output transistors rather than one.
As to the wire gauge, this depends primarily on the length of the runs as shown in
this excellent explanation of speaker wiring by a veteran audio professional. For the M80s alone, 16ga would likely be fine, but the other speakers need to be considered as well.
