A 404 page or more commonly “not found page” is encountered when,
-user clicks on a broken link
-the page has been deleted
-user mistyped a URL
You should try avoiding 404’s since they lead to a bad user experience. You can create a custom 404 page for a good user experience
For more on 404’s, refer to this link,
http://googlewebmastercentral.blogsp...t-my-site.html