I bought the medium and my son is 4 1/2- but he is short/small.  The medium was plenty big, but the legs gather and it wasn't a big deal.  The hat is not as oversized, so i am glad i chose the larger size. It was cold trick or treating, so it was large enough to fit a sweatshirt underneath, so that helped too.
 He LOVED it and had a great time flying around telling everyone he was "a bat, NOT Batman". :)  Others noted color changes, but ours was grey and black, just as the photo shows it.