Gustavo's reply is excellent. I would like to add a minor historical note.
Six hundred years ago, the word "only" was spelled and pronounced "onely", indicating that it could only refer to something that was unique. Back then, had the United team existed, one might speak of "the onely United fan", but "onely four United fans" would be self-contradictory. There was either just one, the "onely one", or there were four (the "fourly four"? Eek!).