I hear break-up in my v2 tweeters that I don't hear in the v4. The off-axis response is also much better. It's one of the reasons I want to upgrade my M80s and surrounds.
Version 4 has the new tweeter and crossover. The new crossover incorporates learnings from the evolution of the family of curves. The tweeter has a die-cast aluminum faceplate instead of plastic for improved rigidity and cooling. High frequency response is smoother due to a new horn design and larger rear chamber. It also handles more power. The mid-range baffle is also stiffer. I am not sure about this but the version 4 may also have die-cast baskets for the drivers. It's also quite possible the cabinet is stiffer.
