When I bought my first set of Axiom speakers I found a VP180v3 in the seconds deal page at a good price and said why not. I really have not put any thought into the speaker since.

I wonder if I am missing out on anything really major? Would it be worth the cost to upgrade to the 180v4 or would the VP160v4 be just as good? Would I see a noticeable improvement or is it just throwing money at the wall for negligible benefit?

I heard the 180v4 and it sounded no different than my 160v4. I can dump 950 peak Watts into my 160 and it sounds composed. I haven't tried to dump more than that via the 1500. The 180 can certainly take more though and go louder.

I can't say enough about the v4 tweeter. I think you will notice a difference there.

I suspect, based on what I know of v3 vs. v4 M3, the cross-over has been improved.

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.