1. Reverted back to more "round" shape (instead of oval) - so back to what people were happy with on the C1/2/3 range.
2. Changed the lining material to be "nicer" and "better" (cooler) in hot weather
3. Reverted to a "real" Pin-Lock instead of the C4 Schuberth "Pin-Lock-alike"
4. Changed the front vent design/operation, to be more robust (the mechanisms were breaking...)
Personally, there were too many things wrong with the C4 for me (included comms kit) and poor reviews, so I just bought a brand new C3 PRO (having had a C1, C3 and C3 in a row).
Having discussed that with Schuberth over email, they have alerted me to the new "C4 Basic", which is a new C4 PRO, minus the integrated comms kit, which would be perfect for my use (I still use an Autocom install). Assuming the C4 PRO reviews (there are not many, yet) sound good, I'll be happy to get a C4 Basic next to replace the C3 PRO, when it's time is up (or the C5/C5 PRO, if indeed that will be the thing next, when the time comes).
HTH!