I submitted 4 letters mostly due to the fact that I got "click happy" while submitting my ERAS application. It was much easier to click all 4 boxes instead of just 3!
In hindsight, it probably didn't matter either way. I've got interviews at all of my top choice programs so if it was a mistake, it didn't hurt too bad. Unless the fourth letter is glaringly weak, it's one more way to help programs evaluate your application. The trick is to send 4 equally "strong" letters.
MHO