404 error is a normal error that the webpage/URL you were trying to reach could not be found on the server. It is a Client-side error which means that either the page has been removed or moved and the URL was not changed accordingly, or that you typed in the URL incorrectly.
for example below webpage has been removed and no longer exist.
if your trying to access the above webpage you will get 404 error .
lets say you get this error when your trying to run any "yum" command it means the repo URL which setup in your repository is no longer exists .