I believe that once you get to the 300K level, you know how much to exert yourself and how to eat for the long rides. The main thing is to keep doing long rides. Doing a century, or longer, ride on weekends should be good to prepare you for the longer rides. If you are riding brevets near home, perhaps there are some 200K permanents that you could arrange to do on the weekend? If nothing else, you could go out and do the 200K route some more.

You might practice night riding since a 400K is going to require quite a bit of riding after dark. Getting a good light setup helps quite a bit.

Good luck on the 400k and 600k rides. I'm looking to do my first 400K at the end of the month.