The 404 error is an error code in the HTTP protocol. This code is returned by an HTTP server to indicate that the requested resource (usually a web page) does not exist.
Some web browsers then display the message “404 File Not Found” (English meaning “file not found”) to the Internet user.
The first 4 indicates an error in the request, here a bad URL, coming from an obsolete page or a web address entry error on the part of the visitor. The last 4 indicates the problem caused by this error: the resource could not be found.
(https://fr.wikipedia.org/wiki/Erreur_HTTP_404)
(English version: https://en.wikipedia.org/wiki/HTTP_404)
