What counts as first four years of college?

I'm looking at claiming the American Opportunity Credit and I'm not sure if I qualify or not.  When they say "first four years of post-secondary education" what does that exactly mean?  I spent about 4 years at a community college because I changed majors.  I graduated from the community college and now I'm pursuing my bachelor's degree.  Since I have not completed my four year degree yet and I have never claimed the American Opportunity Credit, would I qualify?