Customizing your 404 page is not really that hard. But many folks prefer not to touch any code and use plugins to handle everything. These 5 plugins can help you customize your 404 page quickly:
404 to Start: lets you send 404 page not found links to your start page. It offers optional email alerts.
Genesis 404 Page: this plugin lets you customize the content of the 404 page of your Genesis-powered blog.
All 404 Redirect to Homepage: this plugin redirects 404 error links to your homepage. It is a quick fix, so I recommend a more sophisticated link redirection scheme to handle broken links.
All In One WP Security & Firewall: a sophisticated plugin that enforces security practices and logs 404 events on your site, so you can address them.
Should you rely on these plugins to customize your 404 page or page-not-found issues? That’s up to you. Those who are more comfortable with PHP and HTML may want to consider creating their own custom 404 page though.