STOP raising your bounce rates and lowering conversion rates by redirecting customers to an outside 404 page.
With The Cart Designers 3dcart Custom 404 Page your customers get an internal page within your site. This leaves them in a place to go anywhere from your actual menu and/or actual sidebar. You can also modify the 404 page content via Extra Pages to include anything you want.
Use the built in WYSIWYG editor to add sales banners, links, clever 404 comments.... It's all up to you.