My website www.downtownseattlechiropractic.com presently has a PR 4. What is the best way to improve PR?

[Jake]

That is an insanely high PR for a site one tool reports has only 7 external incoming links, such page ranks are sometimes the result of a paid SEO service using manipulative redirect tricks to fool the searh engines.
Google Page Rank is a somewhat dated overall rating system that Google only updates about 5 times a year and I suspect does not use so much internally. It is a useful gauge to appraise sites for sale or that are renting ad space.
Improving search rank position for important keywords is probably much more important, certain searches are known to convert to leads, there are some well known doctor and lawyer terms that competitive bidding in the Adwords sponsored search system have driven up to over $50 per click. Ranking for free organic traffic for some of these keywords is a bargain.

I suggest you concentrate efforts on local ranking, Google has identified searches made locally, often from cell phones often result in sales. Making sure your site looks right on cell phones may payoff as well as chasing the more abstract page rank rating which is already higher than most sites catering to a local clientele, optimizing the Google (as well as Yahoo, Bing) local places page would be well worthwhile, employing a paid business directory submission service to generate more citations in yellow page sort of sites may pay off as well.