What is the purpose of the "fourth son" in the telling of the Pover story in the Haggadah?

The first son is wise. This is to demonstrate that we should all seek out as much information and knowledge as possible and practice what we preach. The second son is simple. This is to demonstrate that we should teach our children only what they need to know at that moment in time and to not bombard them with too much information. The third son is wicked. He is rebuked to demonstrate that each of us is responsible for our own actions and must be cognizant of the actions of people before us. The fourth son, who does not even know how to ask a question, we are told that we must begin for him, but what lesson are we supposed to learn from him? Why is the fourth son in the Haggadah?