After many years, a prisoner is finally released. He runs around yelling, "I'm free! I'm free!"

After many years, a prisoner is finally released. He runs around yelling, "I'm free! I'm free!" A little kid walks up to him and says, "So what? I'm 4."

This is a joke about misunderstandings and speech impediments.

The kid obviously misunderstood the prisoner. He thought that the prisoner was trying to say "I'm three", but had a speech impediment that made him instead say "I'm free."

The kid then disparages the prisoners age, and says that he is older.