tracking down the etymology:
according to Arguing With Signs:
"In February, Daily Pundit came up with the Four Horsemen of the Blogopalypse: Andrew Sullivan, Glenn Reynolds, Charles Johnson and Steven Den Beste."

Max Sawicky refers to them as the 'four horses asses of the ablogalypse' and is also credited with coining it. I must admit I like Sawicky's version better. And I have no idea who Charles Johnson is.