We learn that when one says echad in the Shema they should draw out the word and concentrate on its meaning, One. The four sons represent the full spectrum of Jewish observance from the learned to the wicked to the uneducated. Each are imbued with a special way of approaching their Divine service.
There once was a tzaddik, holy rabbi, who would before beginning his own Seder, walk aound the village and listen how different people they conducted their Seder. As he walked down the cobblestoned alleys of of the village he could hear from all sides the voices of his simple people singing and reciting the narrative of the Haggadah, each in his own way. He once stopped still near the wooden shutters of one of the cottages and heard a voice reading aloud: “The Torah speaks of four sons: one wise, one wicked, one simple, and one who does not know how to ask questions. And every time the reader came to the word for “One”— echad — he would cry it out aloud with prolonged concentration, just as people do when they say the Shema.
The holy rabbi was delighted, and commented later that this simple man had made out of the Four Sons of the Haggadah — including even the wicked son — a powerful prayer, a prayer as holy as the Shema.