Reading on the V4 page - it states that:

'Every crossover in the entire line has been modified to incorporate research discoveries made in the family of curves.'

http://www.axiomaudio.com/version4

From that, I would say that the Xover in the QS8V3 is different from the new QS8V4s.

Personally, I feel that changing out the tweeters would not be worth the expense (more so if Xovers are involved) for the resulting subtle audio change, especially in surround speakers that are used minimally in most movie mixes anyway. Thankfully, I'm happy with my V3s as they are...

TAM