@shopfrontsigns, PR doesn't depend on traffic, it depends mainly on how many inbound links you have (although I admit, the two are correlated).
I have a feeling that the reason your wp.com blog has a PR of 4, is because of all of the inbound links you are getting from wp.com. But that's just a guess. If it makes you feel better, I would like to actually know what my pagerank is, but different systems seams to give me different pageranks.
Anyway, I have said this before on some other blogs, PR is not the end all be all of SEO. PR is just one of the things google uses to determine how high it should place you. More importantly is having a good file that Google's spiders can read. The good news, wordpress.com does a good job of that.