For a 3-4 years old, an Oyster 3 footmuff - nor any other, to be honest - will not be enough to really cover the child. At such an age where the kid is supposed to walk more than to sit (or at least go from one to another often), a footmuff is rarely necessary- usually just good winter clothing + a blanket is enough. Definitely would not get the Oyster 3 footmuff here, rather go to a stroller store and try a few out, but you might end up rather getting a thick blanket and good overalls for you little big one.
P.S. I quite liked the Lodger footmuffs, and they fit even a 3 years old, BUT it was a bit of a tight fit. The larger Lodger Explorer one that is already discontinued was even larger; try checking classifieds.