The answers provided to the four sons are found in four separate parts of the Torah. Four times we are told to relate the story of the Exodus to our children, and in each place we are told to tell it differently in response to a different style question (or in one instance, no question at all). The authors of the Haggadah, therefore, understood these passages as guidance in relating to four types of children.
Here are the sources in the Torah:
The wise son is found in Deuteronomy 6:20-23. Note the lengthy and detailed question and answer:
If your son asks you in time to come, saying, "What are the testimonies, the statutes, and the ordinances, which the L-rd our G‑d has commanded you?" You shall say to your son, "We were slaves to Pharaoh in Egypt, and the L-rd took us out of Egypt with a strong hand. And the L-rd gave signs and wonders, great and terrible, upon Egypt, upon Pharaoh, and upon his entire household, before our eyes. And he brought us out of there, in order that He might bring us and give us the land which He swore to our fathers..."
The wicked son is addressed in Exodus 12:26-27. Note the aloof terseness of his question and his sarcastic tone: "What is this service to you?" removing himself from the matter:
And it will come to pass if your children say to you, "What is this service to you?" You shall say, "It is a Passover sacrifice to the L-rd, for He passed over the houses of the children of Israel in Egypt when He smote the Egyptians, and He saved our houses."
The simple son's question, and the appropriate simplistic response, is found in Exodus 13:14. Note the simple exchange:
And it will come to pass if your son asks you in the future, saying, "What is this?" You shall say to him, "With a mighty hand did the L-rd take us out of Egypt, out of the house of bondage.
The son with whom the parent must initiate the discussion is addressed in Exodus 13:8:
And you shall tell your son on that day, saying, "Because of this, the L-rd did [this] for me when I went out of Egypt."
The Rebbe, Rabbi Menachem Schneerson, taught that today there is a fifth son, the one who is not even present at the Seder; completely unaware of his great and proud heritage. It is our duty to proactively seek out these sons and daughters, invite them in and share our beautiful traditions with them as well.
Click here for more about the Four (five) Sons.
I hope that I've been helpful today.
Rabbi Menachem Posner