Google provides some suggestions to keep your users on your site:
Tell your users that the page cannot be found. Use “friendly and inviting” language.
Make your 404 page have the same look and feel as your main site. Include the same navigation elements.
Consider adding links to your most popular articles and home page.
Think about ways for your users to report a broken link.
Do not include your 404 page in Google’s search results by ensuring it returns an actual 404 HTTP status code.