The number 404 refers to HTTP error code (404) that is returned by the server, when URL user asked for couldn't be found ("Not Found"). In joking or metaphorical manner it is used as a synonym of "not found", "unavailable", "non existent" and so on.
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404 Error: Not Found. For more information, please visit our homepage at h t t p s : / / w w w . y o u t u b e . c o m / w a t c h ? v = d Q w 4 w 9 W g X c Q
Normally an error given when indicating that the client was able to communicate with the server, but the server either could not find the file that was requested, or was unwilling to fulfil the request for it and did not wish to reveal the reason why.
More recently used to describe a user or individual that, at any given moment will lock up and seem to have no intelligence what-so-ever and will not co-operate to solve an issue. (ie No Intelligence Found)