404.php shows if you have a permalink problem. Can be a htaccess problem or a third party plugin affecting th URLs. To check, follow these steps:
1. Go to Admin – Settings – Permalinks – set DEFAULT save and check again. Does that solve the problem?
2. If yes, go to the same settings and select CUSTOM permalink. Do you get a notification that htaccess needs to be updated? if yes go and update htaccess (wordpress file) and set the permissions via FTP. WordPress help – http://codex.wordpress.org/htaccess
3. If you don’t get htaccess message, check now. Perhaps you installed a plugin that reset permalinks and this will update the links rights.
4. If this doesn’t work either – disable all your plugins, reset permalinks and see if that helps.
Last resort, email me wp-admin credentials to do the above.