If you are getting 404 error when accessing pages, please see the information below.

Symptom: 


I just setup my WordPress installation and my theme. I went into Admin -> Settings -> Permalinks and selected something other than the default option. But now when I try and navigate to a page, it just returns a generic “404 Error – Page not found” message. 


Likely causes:


First, this is not a theme issue. The problem involves your core WordPress installation, or your web server configuration. That said, there a few things that can cause this to happen:


Apache's mod_rewrite module is not being loaded by your web server.


Your WordPress .htaccess file is missing or not writeable by the web server.


Apache is configured to disallow overrides by your .htaccess file.


Solutions: 


There are several solutions to this issue, please see the list below.

First make sure that mod_rewrite is installed and enabled on your Apache web server, and also make sure that the web server allows your .htaccess file to override the server directory configuration.


Make sure that your WordPress .htaccess file is present in your WordPress installation. If it is not, simply create a blank file called .htaccess and upload it to your WordPress installation folder.


Set permissions on your .htaccess file that allows your web server to write to the file – CHMOD 666.

Go back and set your permalinks option to the “default” option and save the change. Then set it to something other than the default and save the change.

If your .htaccess file is writeable by the web server, it will contain new directives that sets rewrite rules for your custom permalink option.

If it is not writeable, you will see a message below your permalinks screen that contains the rule set you need to add manually into your .htaccess file, save it, and upload it back to the WordPress installation folder.

Check to see if you can navigate from your front page menu links without error.

If the issue persists, contact your web host support team for further assistance.


source:http://theme-fusion.com/knowledgebase/custom-permalinks-return-404-errors/