I can't imagine finding anything suitable for whitewater for $400, unless it is older than you are and full of holes! LOL
Check out the classifieds on this site to get an idea, most of what is sold on this site are true whitewater boats. Most of them will run you considerably more than $400, but the boats in good condition are going to last you a long time, and are going to be fit for purpose (ie, whitewater).
You'll spark a debate of Hypalon vs. PVC, and everyone has their favorite brand, and even size. I went through all of this in the spring, and I spent a lot of time checking out the manufactures Web sites, looking at the ads here, reading some of the threads in the Gear Talk forum on this site and talking to friends and guides (go on a guided trip as a first step).
If there's a shop anywhere near you, go in and take a look at a few boats and ask them questions too. They'll obviously tell you what they sell is the best stuff LOL, but if you do enough research, you'll be able to tell if what they have meets your needs.
You'll also need to budget a LOT more then you think, because the boat is only the beginning! Paddles, PFDs, helmets and a lot of other gear adds up quickly, not to mention a good size cooler for all of your beer...
Good luck with your search! Gearing up is half the fun!