Help! First Time Home Buyer credit?

I bought a home for $128K in 2009. Amended my 2008 return to claim the credit. Married filing separately. But I bought the home with a co signer who is not my spouse. I only claimed $4000, but I believe I was eligible for the full $8000 since I was the only buyer eligible for the credit and I didn't buy the home with my spouse. If I am eligible for the full $8000, how do I go about getting the other $4000 since I already filed an amended 2008 return to claim the first $4000.