Not sure how acurate this info is, but it's worth a post.
Check inside the frames seat tube to determine if there's an aluminum sleeve present. If there is, it should be 4 inches long.
Because certain carbon frames seat tubes flex while riding, you have to be sure the seatpost does not extend very far past the bottom of the sleeve. Otherwise, the bottom edge of the seat post MAY rub against the carbon seat tube if the seatpost is set really deep into the frames seat tube.
Does that make sense?
I'm guessing the info applies to really long 400mm seatposts.