the reason i know for the four fingers instead of five is mostly becoz its easier to draw 4 stubby fat fingers than 5 slim and dexterous fingers especially for animation where your draw the same character repeatedly, this is just my experience though
Actually, part of it has to do with the fact it is easier. In Japanese manga, this is usually avoided because a common yakuza punishment was cutting off a finger. So if a mob sign was cutting of a finger, we would probably see more 5 fingered folk.
the finger thing it's an interpretation of a study (i cant remember from which american university) where a genetist... or just a doctor, found that the men who have a natural tendency to develop muscle and strong "manly" traits, as body hair, deep voice, great reflexes and fast speeds were to the ones that have the ring finger almost the size of the middle finger. This study relates via empiric measurement the relative size of both fingers with the amount of testosterone received by the fetus while at mother's womb. The testosterone received while in womb is what change the "sex" (because we all are genetically women 'til the sixth or thenth week, i think) and then is when we begin to delevop a penis.
The study was based in a lot of measurements taken from thousands of athletes at colleges and universities, they found that most of times, you can predict the athletic performance just by looking the hands...
Actually, seeing as how that's a sensitive organ it's only natural for Dragons to keep it hidden. Also, cartoon logic within a 4koma about a cartoon? That pretty much makes Pinkie's argument to Spike null.
And since Ponies in the show have hooves and NO fingers, well you get the idea.