400 refers to the spine of the arrow, not the weight. Most everything I've heard says shooting a little on the stiff side with carbons isn't a huge problem normally, especially since they tend to weaken a bit as time goes on. The charts I have say you should be shooting between a 300 and 350, 350 being the stiffer side of the spectrum. To be honest however, if you're shooting them well, I wouldn't change anything.