Just like shabbir had said, Yahoo concentrate on on-page SEO while Google in backlink.
Your site had a PR4. But it doesn't mean you have enough backlinks to compete with other web sites who has the same keywords with you. As you can see, there are millions of web site which has PR0-4.
To me, your site might be :
1. Sandboxed - If you have doorway pages or links management system which artificially create links
2. Not optimized - For example: (a)if you build a forum under ezboard subdomain, you can easily archive PR4 but it wont get a good position on Google. (b) When you create an online store using getestore.com, you also easily get PR3-4 in 2-3 months time, but once again you wont rank well for your keywords.
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Case study
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Visit this two different site which has PR 0f 3 and 7, but yet doesn't rank well in Google
http://www.ausman-mall.com - Using getestore.com
http://p090.ezboard.com/brakpakman - Using ezboard
 Google knows what you do 
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