The Four of the Apocalypse

We all know about the Four Horsemen of the Apocalypse: Death, War, Famine, and Pestilence riding on pale horses and all that Book-of-Revelation stuff. But why does it have to be four guys on horses? Why not the Four Cheerleaders of the Apocalypse? The Four Cats of the Apocalypse? The Four PTA Moms of the Apocalypse? The Four Lawyers, Librarians, or Lunch Ladies of the Apocalypse? The Four Drummers, Rock Stars, or Opera Singers of the Apocalypse? Or even the Four Squirrels of the Apocalypse ...