Jackie's very correct. I just want to add that:
Dogs, especially puppies do not recognize children as authority figures. Your daughter, trying to discipline him and only being 4 years old, is not a good idea. I've seen kids that young tell dogs "no" and then get angry at the dog as if another child for not listening. The child then "chastises" the dog the same manner as being a pretend-parent-- either by smacking, getting loud, stamping, or getting upset.
Not good because neither understands what's going wrong. They are about the same age in maturity at the moment, despite the massive size difference :)
When he starts nipping her, separate both of them and stop the play immediately. She might be instigating by pulling on tail/ears or running around thinking that's it's fun which he'll respond to in puppy way :rolleyes: I did when I was that age --many kids do, it's their nature to tease their friend ;) :) You just don't know what's going on when you're not looking.
Remember, just because he's a large dog, doesn't mean that he's as mature as he looks until 2-3 years old.
good luck,
toodles