Since you're an SEO blogger you'll know all of this but for those figuring the question above:
Rule number one - while PageRank is still visible - is content. Original content, posted often enough to interest Google in viewing you as a potential authority site, which essentially what a PR4 is.
So post often - every other day at least, two or three articles of around 250 words.
Get backlinks where you can - decent zoned-in on your authority niche. Join the forums and promote you and your website as authority figures. People will want to backlink to you.
Join WebMaster Tools at Google, use analytics on the site and - this is a good one - use Google Search on your site. Get involved with Google. Look at providing RDF feeds of your site - Google is rapidly moving to RDF-a as an opportunity to start mapping the real-time web.