I do see the point of the question.
First, I don't know if there is a rule/requirement that everybody becoming Canadian has to be in Canada.
From the practical point of view, this must be considered: at the ceremony, you have to surrender the PR card of everybody who becomes Canadian. You must surrender the PR card of the 4 year-old.
If the 4 year-old is in the US at the time, the question is how s/he will return to Canada, and under which status. If the kid is taking a plane to go back to Canada, this will only be possible with the passport if s/he is from a visa-exempt country and if s/he carries the citizenship certificate. If the kid is returning by land, then s'/he can do so with the citizenship certificate alone. Either way, you must find a way for the citizenship certificate to reach the kid and for the kid to return by land if s/he doesn't have a visa-exempt passport.