Tony,
All but one of the guy's around here used the C-4 because of the lighter weight. The drag races use them and have them beefed up inside. One friend used the C-6 and has wished ever since that he had gone with the C-4. I think that weight is the big difference and you can have the C-4 beefed up inside to take a lot of abuse. As for the flex plate I leave that to the guys that know more than I do about this.
Ron
