Meaning of 404
When we expand the code 404, the first digit “4” represents a client
error. The server indicates that you did a mistake like misspelling the
URL or requesting for a page that is no longer available.
The middle digit, 0 represents a general syntax error and could
indicate a spelling mistake.
The last digit, 4 refers to a specific error in the group of 40x.
The World Wide Web Consortium (W3C) states that 404 Not Found should
be used in cases where the server fails to find the requested location
and is unsure of its status. Whenever a page has been permanently
removed, the status code used must be 410. But hardly have we seen a 410
page. Instead, 404 Not Found page has become popular and the most
commonly used error page.
Content of a 404 Error Page
A 404 response code is always followed by a human readable reason
phrase as per the HTTP specification. Generally, a web server issues an
HTML page that has the 404 code and the “Not Found” phrase by default.
You can configure a web server to display a branded page with a better
description and a search form. But the protocol level phrase requires no
customization as it is hidden from the user.
Soft 404s
Soft 404 errors are actually “Not Found” errors returned by a web
server as a standard web page with a 200 Ok response code. In an
automated process of discovering a broken link, the soft 404 errors are
problematic.
The BT Group of UK has a clean feed content blocking system that
returns a 404 error to the requests for content identified as illegal by
the Internet Watch Foundation. Even when the user tries to access the
Government censored websites, a fake 404 error will be returned.
I have found good article from below link for this can you please go through below link :
http://www.404errorpages.com/
http://www.checkupdown.com/status/E404.html