A 404 error typically indicates a URL or Permalink problem, but can be caused by other things. Check your WordPress Custom Permalink Structure. These links below contain other common issues/problems that can cause 403, 404 or other errors.
Incompatible Web Hosts
http://forum.ait-pro.com/forums/topic/read-me-first-free/#incompatible-hosts
cPanel HotLink Protection Tool Problem
http://forum.ait-pro.com/forums/topic/read-me-first-free/#cpanel-hotlink-protection
WordPress flush_rewrite_rules function problem
http://forum.ait-pro.com/forums/topic/read-me-first-free/#flush-rewrite-rules
DSO Configured Server
http://forum.ait-pro.com/forums/topic/read-me-first-free/#dso
BulletProof Security WP Error: “no input file specified”- Permalink Problems/404 Errors – using the .html permalink hack is causing 404 Errors
http://forum.ait-pro.com/forums/topic/read-me-first-free/#no-input-file-specified
403 Forbidden Error after updating/upgrading BPS
Hosts that do not allow root .htaccess file 404 file permissions and general troubleshooting steps for this problem
http://forum.ait-pro.com/forums/topic/read-me-first-free/#root-htaccess-file-lock
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