I know very little about epilepsy, but a very rudimentary search gives results that suggest you can get your 400mg per day over the Internet for a tolerable price, 60 to 113 cents per day:
(60 cents per 400mg)
http://www.canadapharmacyonline.com/DrugInfo.aspx?name=Dilantin0376&RefID=froogle
(113 cents per 400 mg)
http://www.drugstore.com/pharmacy/prices/drugprice.asp?ndc=00071036932&trx=1Z5006
I presume that there is no special magic about a 400mg pill versus four 100 mg pills, of course.
Hope this helps.