Find the missing number

The Fourth of July was coming up, and the nursery school teacher took the opportunity to tell her class about patriotism. "We live in a great country," she said. "One of the things we should be happy about is that, in this country, we are all free."
One little boy came walking up to her from the back of the room. He stood with his hands on his hips and said, "I'm not free. I'm four."