I have a relative to takes them, and often can't find them in the right dose. She uses the 200 mg pills. As long as it adds up to the dose you should be taking, you'll be fine. If you're going to a pharmacy, ask them to order the 400 mg tablets for you, they probably will. Small town vendors are usually willing to go the extra mile to keep their customers happy.
Good luck,
Greg
"Beauty is truth, truth is beauty - that is all you know on earth, and all you need to know"