You might as well say that the credit was claimed for 4 years, because you are no longer eligible for the American Opportunity Credit anyway. If you have completed 4 years of college you are not eligible for the American Opportunity Credit, even if you didn't claim it for those 4 years.
Didn't TurboTax ask if you have completed 4 years of post-secondary education? If it didn't ask yet, that question might be coming up.
