Don't worry about it. There hasn't been a toolbar PR update for a few months so if you have been building good backlinks to your site then it will still have PR - it just won't show in the toolbar.
Sorry, i just now come back and see your reply. I used my other PR4 site to link it，more than 5 links. All of those back links were only one-way, and this site only linked to a small quantity of high-quality site, like imdb.com/douban.com/mtime.com.
Hope that was not a dropped domain. Because some times dropped domain with pr lose pagerank when you renew and in the next pr update you can the pr back. or maybe you must have some strong pr 5 or higher backlink which gave you a pr 4. But google doesnt do update for one domain alone. Its all the good time you had [] I had a pr 5 domain and in a update it became pr 3 then became pr 0. Now the PR is 1. I have a strong pr 6 backlink. But still no increase in PR . []
I don't understand the thread. How can you get a PR4 in 30 days if Google doesn't even update every month?
I had a free hosted blog have PR4. I put it on it's own domain and hosting and got a PR 4 on that on the first update.