It is likely that actual multiples can look different. Twins can be monozygotic twins or dizygotic twins. The twins we normally think about as twins, which look identical, are normally monozygotic twins. They split from a single fertilized egg. that makes them look identical since they share the exact genetic info. However, two eggs can be fertilized at the same time by different sperms, leading to two babies born at the same time. These babies, however, share non-identical genetic info and can only be considered as normal siblings, in terms of genetic make-up, except born at the same time. The most obvious non-identical twins in the latter case would be the "dragon-phoenix twins", a twin of a boy and a girl. If both babies don't even share the same sex, they have to be non-identical twins from different fertilized eggs. They would be no different than any other brothers and sisters, except they were born at the same time. this is also true if both babies are the same sex, just the same as any other same-sex siblings, except they are born at the same time. In this case, they normally look different, just like any other siblings.
So in the case of the quad, it is possible that the mother had 2 eggs fertilized at the same time (this happens a lot especially if the mother underwent fertility treatments) . One of the eggs eventually splitted to give 3 identical babies and the other egg would become the big guy. So they can still be considered as multiples in the same sense as the "dragon-phoenix twins".
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