I'd ask Axiom if besides the tweeter, the v4 has a more refined presentation (i.e. the holography I keep babbling about). I wouldn't expect the M5 to sound better than the M80v3, especially with the new tweeter, but then again, who knows? Then there's your room. The smaller driver complement of the M5s may provide for an improved room interaction.

House of the Rising Sone
Out in the mid or far field
Dedicated mid-woofers are over-rated