I think its stems to when Channel 4 used to show PPVs. The big 4 used to be Box Office and the others Sky Sports, but Channel 4 had about 4 PPVs? The only one I remember being on there though was Backlash which is now Extreme Rules. So now the big 4, plus any that used to be on Channel 4 are now Box Office.
Yeah i remember back a good few years ago watching the Royal Rumble on Channel 4 jesus that was years ago now i was 4 at the time!!