Good question! By speaking of the four types of children — wise, wicked (or wayward), simple and ‘one who doesn’t know to ask’ — the author of the Haggadah teaches us about the responsibility and privilege that each generation has in its mission of Jewish continuity.
This is the significance in the Hagaddah of speaking about the four sons. The author hints at the danger of a lack of education by his unique order of the Torah's four sons. He feared degeneration from monotheism to self-worship (a form of idol worship) — a path opposite from that our ancestors traversed.
A wise child who asks questions demonstrating a basic knowledge of Judaism and is not answered properly may be so bitter that even if he himself is observant, his child will move away from Torah and mitzvahs.
This wayward second generation will refuse to educate the third one.
This Jewishly-simple third generation will never understand the parents’ rejection of Judaism. He will be curious, but not overly interested in his heritage.
He will produce a fourth generation which feels that the Torah could not possibly be intellectually satisfying. He is therefore so far removed from Torah that he has no interest in participating actively, nor does he know how to begin investigating.
If he does not unearth the depth of Torah, the fifth generation will not even attend a Pesach Seder…