Basketball fans are called the “sixth man”, and I believe the Orlando Magic have retired the #6 in their honor.
It was already mentioned that Andre the Giant was called, by the WWF, the “Eighth Wonder of the World”. Well, later came Chyna, and she was named the Ninth Wonder of the World.
I only just learned of Murray the K from a NYTimes crossword puzzle a few weeks ago. It was a “fifth Beatle” clue, and none of the answers I normally associate with the Fifth Beatle (Epstein, Sutcliffe, Martin, Best, Preston, etc.) fit in the space. I have even watched the 8-part Beatles Anthology and somehow either missed him or just completely didn’t file him away in my memory banks. I wonder if that’s one of those things you would know if you were an American growing up during the Beatles craze vs. a generation afterwards (like me.)
A group that undermines a larger group is called a Fifth Column, usually in favor of an enemy group or nation. It was claimed that Japanese Americans living in Hawaii helped the Japanese.
Taco Bell, long a haven for late-night inebriated dining, ran an ad campaign a few years ago, positioning those late-night taco binges as the “fourth meal,” after breakfast, lunch, and dinner.
Newspapers have been called “The Fourth Estate” for 200+ years.
It comes from just before the French Revolution: there were three “estates” in their legislative body: the church, the nobles, and the commoners. Laws required two estates to approve. The power of the newspapers in influencing public opinion led them to be called the Fourth Estate.
Tommy Oliver, the Green Ranger, was the “Sixth Ranger” on Mighty Morphin’ Power Rangers, and “Sixth Ranger” is now the TV Tropes name for the phenomenon in which someone more powerful joins an established team.
While not quite as influential as newspapers, and certainly nowhere near 200+ years old, CBC-TV in Canada has had a news program for years, called The Fifth Estate. It’s for televised investigative journalism, and built its name off the traditional Fourth Estate.