K.B.
If I was the mother, and it was MY 4 year old and MY newborn, I would be telling the 4 year old to simply knock it off - there's no reason for him to be poking the sleeping baby in the face, however gently he might be doing it. But if I was you, and it was not my kid doing it, I probably would not say anything, only because generally other parents don't like people besides themselves correcting their child.