Nah, the 400 meters takes real work. It's not exactly a sprint and it's not exactly a distance you have to pace yourself at, but you do need wind. The way to practice for the 100 is to run 200's, and for 200's to run 400's, but for 400's you need to run 800's and that is one tough distance.
Hell, the 800 is practicaly cross country for a 100 meter sprinter.