Memory Problem

ok so i own a white macbook that i got at Christmas, 2007. i just purchased 4GB memory from OWC (2- 2GB modules), and upon installation nothing happens. i can hear the hard drive engage, and my power light on front turns on, but my display stays black. when i take out one of the modules and put in a 512mb card that i had just taken out, the computer boots up fine, and the same when i put in the other 2GB card and a 512mb card. i was under the impression that the model i had can support 4BG ram... did i do something wrong?