Who are the Four Sons ?
Since the Passover or Pesach festival and Seder are annual events, the Four Sons symbolize and represent a model for transmitting the story, rituals, and messages of Passover or Pesach to future generations. The Four Sons also represent an educational tool for responding to the reactions of different types of personalities to the Seder and its rituals based on how each of these different types of personalities views the world. According to this model, there are four types of people, each represented by a son: (1) the Wise Son, who wants to know the details of all the Passover or Pesach rituals (in Devarim or Deuteronomy 6:20, his question is asked; the answer to his question is in Devarim or Deuteronomy 6:21-25); (2) the Rebellious Son, who disowns all the rituals of Passover or Pesach (in Shemot or Exodus 12:26 his question is asked; the answer to his question is in Shemot or Exodus 12:27); (3) the Simple Son, who asks questions in a simple way (in Shemot or Exodus 13:14 his question is asked; the answer to his question immediately follows his question and is also in Shemot or Exodus 13:14), and (4) the Son who is so young he does not know what and how to ask, and so he cannot ask [since this type of Son cannot ask, the father must begin for the son, as the Torah states "and thou shalt tell thy son in that day, saying: 'It is because of this ("this" being the matzo and maror which has been placed before the Son on the Seder table before the father teaches him this statement) that the L-rd acted for me when I came forth out of Egypt'" (Shemot or Exodus 13:8)].
Once the Four Questions - in reality, not Four Questions but one main question and four clauses - are asked by the youngest member at the Seder table, the Four Sons are listed as told in the previous paragraph and their way of understanding the world is described as also told in the previous paragraph. Following this, the father of the youngest member answers each of the four clauses to the youngest member according to which of the Four Sons the youngest member represents in terms of the youngest member's understanding of the world. If it is determined that the son represents the Son who is so young he does not know what and how to ask, then the answer to this is found in the Talmud, in Mishnah Pesachim 10:4, where it states that if the son does not know what to say, his father teaches him what to ask. As also detailed in the previous paragraph, the father answers his son by teaching him the statement or statements from the Torah that are appropriate for the way in which the son understands the world.
The purpose of categorizing different understandings of the world from the point-of-view of a child into Four Sons is to ensure that whichever understanding of the world the child possesses, his father will be able to teach him the tradition and commandment of commemorating and celebrating Passover or Pesach according to his understanding of the world. This will enable the child to carry on this tradition into adulthood and then pass this tradition and commandment down to his children by, in turn, teaching them according to their understanding of the world, and so on through the generations.