The v4 mid-woofer is different from v3.

I'm speculating but the MDF may have been changed as well at some point. The M3v4 MDF appeared different to me from the Ti I remember. I was working from memory though and it's been 12 years since I last saw and heard the Ti.
A-LFR/1000-8/500v4x2 coming
100,160,QS10x2,800,M5,M3,M50 v4
had v2
Air N2 (someday)