While the 40mm watch is 0.7mm wider than the 38mm and the 44mm watch is 1.6mm wider than the 42mm (in aluminum), the lug sizes for the bands were kept the same size. So yes, they fit.
You can actually see the difference in case width if you go to the watch comparison page on apple.com. Look at the curve of the case by the lug and you'll see the series 4 curves ever so slightly out more than the 3 at the same spot. I personally think it looks better.