More than likely, these 404 errors are caused by broken links to your site. If you changed your permalink structure, tags or categories, then this can happen.
Here is a WP plugin you can use to help fix these errors:
http://ww.wp.xz.cn/extend/plugins/redirection/
It is very important that you fix these, since it will effect your site performance, SEO and rankings.
Hope this helps.
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