Tyically a 404 error means that either you are using invalid code (not allowed) for your particular Server/website, invalid code in general, an invalid custom permalink structure or using the .html permalink hack or 1 or both of these problems in the links below is causing the problem.
cPanel HotLink Protection Tool Problem
http://forum.ait-pro.com/forums/topic/read-me-first-pro/#cpanel-hotlink-protection
WordPress flush_rewrite_rules function problem
http://forum.ait-pro.com/forums/topic/read-me-first-pro/#flush-rewrite-rules
Also please watch the Custom Code Video Tutorial to ensure you are doing the Custom Code steps correctly: http://forum.ait-pro.com/video-tutorials/#custom-code