Four Sons

Only three of the Four Sons' questions are mentioned in the Torah (which all related to the Passover sacrifice), since the fourth (smallest) child cannot ask questions. Regarding this child, the hagada says, "Aht p'tach lo" (in the feminine).
Reason  The mother is supposed to be a child's primary teacher while the child is young.
There is a difference in attitude between the wise son and the evil son: The wise son says "Eloheinu" (OUR God); he is asking a question and seeking and answer. The evil son makes a statement (sort of a rhetorical question), instead of asking a question for which he seeks an answer.
Why is there a “wise” (chacham) son and not a “righteous” (tzadik) son, which would be the logical counterpart to the “evil” (rasha) son? 
We don't know who is a tzadik. We cannot be sure by external appearance or even by seeing certain behaviors. We can tell who is a chacham by hearing him speak or by discussing Torah with him.
Yet, regardless of the sons' level of observance, it is a commandment to teach them about going out of Egypt.