Joelwhite got most of the items; to make the list as complete as I can off the top of my head:
(again, these are more B4 parts that are NOT used):
rear control arms
rear inner hinge pins
On the original X - 6, you don't need the B4 servo mounts. On the soon-to-be-released X - 6 Squared, they are used.
X Factory R/C - Viper R/C - Futaba - Darkside Designs - Lunsford - MIP - B Fast Performance Diff Parts