The best method is to just make your 404 content area a widget area instead. This way, you can just edit it from your widgets panel.
This is done in the Hybrid theme if you'd like to test it out:
http://wordpress.org/extend/themes/hybrid
Or, you can add this to your current theme's functions.php file:
<?php
register_sidebar( array(
'name' => '404 Template',
'id' => '404-template',
'before_widget' => '<div id="%1$s" class="%2$s widget">',
'after_widget' => '</div>',
'before_title' => '<h3 class="widget-title">',
'after_title' => '</h3>',
) );
?>
Then, add this to your theme's 404.php file:
<?php dynamic_sidebar( '404-template' ); ?>