Please put your mind at ease. Is she gaining weight? Does she seem satisfied after a feed? Does she last the 4 hours without complaint (at least most of the time)? When you moved to the 4 hour, she probably became a more efficient eater (lucky for you). My daughters were/are the same way and I dubbed them "super suckers". I was super worried the first time around, but came to enjoy that she ate so quickly. As long as she's happy and continues gaining weight, don't worry.
~Rachel