Allows you te set a page to be used as the 404 page. In order for the plugin to work you need to make some modifications to the <code>404.php</code> file in your theme. I set it this way so that the 404 page could be formatted to fit the theme, or can be set more dynamically.
For more information and sample code: http://www.reitzdesigns.com/2010/08/dynamic-404-page-wordpress-plugin/
Although this plugin will work with WordPress 2.8, it’s better suited for version 3.x.

Dynamic 404 Page

Allows you to select a page to use as the content of your 404 page.

Bill Erickson

Gary Jones

Customizing the contents of the Page Not Found page in any WordPress theme can be tricky, and usually involes editing the 404.php template file. The Genesis Framework already abstracts the default content of the page into a function, and this plugin can unhook that and replace it with your own custom title and content.
Use [genesis-404-search] shortcode to add a search form to the page.
If you’d like to dynamically list content (ex: most recent posts), I recommend you install the <a href="https://www.wordpress.org/extend/plugins/display-posts-shortcode/" rel="nofollow ugc">Display Posts Shortcode</a> and use it in the 404 page’s content.

Genesis 404 Page

Customize the content of the 404 Page within the Genesis Framework.

Dynamic 404 Page vs Genesis 404 Page