I ordered both a size 2 and 4 for a tie-dying project for grandkids. I could tell when they arrived that they ran small, and because of the cotton rib-knit, after washing, they looked even smaller. They are actually the same length, but the 2 is a bit smaller than the 4 in the armholes and the shoulder width. My 13 month old grandson, who usually wears 18-24 month size, couldn't get the 2 over his head, and the 4 fit him nicely. I also ordered the 3 pack of Luvable Friends tanks in the 18 months size (those are not ribbed and are cut fuller), but they're actually bigger than the size 4 in this brand. They seem to be nicely made, but please be aware to order larger sizes than normal.