The Chida - Rabbi Chaim Yosef David Azulai - in his commentary "Simchat HaRegel" on the Haggadah, explains that there are three ways one can fulfill the Mitzva of the telling of "Yetzias Mitzrayim. Ideally, the story should be told in the form of question and answer. The Talmud derives this from the Torah's description of Matza as "Lechem Oni" - the bread over which a person answers.
The second level is to tell the story even if nobody asks. This is derived from the verse "and you shall tell your son on that day..." You should tell him, even if he doesn't ask. Thus, the procedure of question and answer is preferable, but not absolutely necessary. (This is a rare example of L'chatchila and B'dieved in a Torah Mitzva.)
The third level is this: Even if a person is alone, he must speak about the going out of Mitzrayim. This is what Rabban Gamliel teaches when he says that one must "say" three particular statements as the bare minimum to fulfill the Mitzva. Thus, there are three possible levels on which to perform the Mitzva of "Sippur Yetzias Mitzrayim."
The Chida adds: When introducing the fourth son The Haggadah uses the word "and." This teaches us that even if someone has other sons that fit into the first three categories, he should also pay attention to the one who does not know how to ask. This is an important lesson for those who are tempted to make the Haggadah an intellectual display which goes over the head of the youngest or least knowledgeable.
