The URL is the world wide web address you are trying to reach. If you typed the URL directly in the Location block at the top of the netscape screen, check it carefully and verify that it is correct.
Common errors involve the tilde key and switching backslash and forware slash.
Some URL's are case sensitive and the capitalization must be copied exactly to reach the site. Most of the time, the 404 message is the same thing as a busy signal on your telephone. The server at the other end was busy sending packets of information to other people who had clicked into the site when your request came in. If the server is really busy it can sometimes miss a request. Frequently you will get right through.