Four-O-Four is the most frequent error among all others. Those HTML errors are used for different purposes. Let me give you whole range of error codes with what actually they mean?
100 : Continue
101 : Switching between Protocols
200 : OK > Action Completed Successfully
201 : Created > Success Following a POST command
202 : Accepted > The req. has been accepted for processing, but the processing has not been completed.
203 : Partial Information > Response to a GET command, indicates that the returned meta information is from a private overlaid web.
204 : No content : Server has received the request but there is no info to send back.
205 : Reset Content
206 : Partial Content > The Requested file was partially sent. Usually caused by stopping or refreshing a web page.