Orkeosaurus wrote:"Pestilence" is actually a bit of a misnomer. The rider on the white horse represents conquest, not disease.
Those can be connected, however. Case in point, the Spanish vs. the Aztecs.
I think four was just chosen because it's a very solid, even number that gives you options without giving too many. Four Gods also means they can each have one rival. And even if this probably didn't figure into
GW's logic, 4 is an unlucky number in a few cultures (Notably Japanese).