Here is a simple way to have WordPress direct any WordPress 404 pages to your main page or anywhere else you like.
Simply edit your 404 theme file, (eg ./wp-content/themes/
Replace all lines of code with the following:
<?php header("Status: 301 Moved Permanently"); header("Location:http://www.YOURSITE.tld"); ?>
However this will NOT redirect a non wordpress page (eg. www.yoursite.com/blah3333/) . For this, you need add an .htaccess file to your root web folder with the following code:
ErrorDocument 404 /index.php
I am not sure if the above .htaccess code would work on Windows based servers.