I have a 2003 Accord, had the same problem and came across this site. I went to the website posed by Taelin. I was able to get the 4 digit for the Nav, but not for the radio. I entered the right number in and it kept telling me it was wrong and finally locked me out after three tries. I found an easier solution though.

On the glove box, there are 2 stickers, one is for the radio, the other is for the nav. You can compare the full serial code of the nav in the trunk to the sticker on the glove box. That way you know which sticker belongs to which piece of equipment.

The 4 digit number on the sticker is the code. You do not even need to go to the website. Just look at the sticker, find the small number and there is your code.