The 16 expressions of forgiveness

The four sons as the Lulav and essrog etc.

The arizal explains on the Hagada that the son "who doesn't know how to ask" corresponds with the arava (as well as the fourth cup) if we extend this allegory of the 4 kinds to the other sons it might go like this:

The wise son corresponds with the Esrog, having both wisdom and good deeds taste and smell. He corresponds with the first cup, the kiddush since he expresses a full measure of sanctity.

The wicked son corresponds with the hadasim, that has the smell of discussion and Torah study, being involved in the conversation and placed next to the wise son, but lacking in good deeds or morals.

He corresponds with the second cup, over the hagada, wisdom on its own without deed.

The simple son corresponds with the lulav, simple sincerity in fulfilling the mitzvos, with out great Torah learning. This type of service can reach up the highest... He corresponds with the third cup, over birkas hamazon. Gratefulness, straightforward praise and the fulfillment of a mitzvah de'oraisa.

The son "who doesn't know how to ask" corresponds with the arava, not even the basic involvement in the discussion, with no knowledge of practical application or theoretics. He corresponds with the fourth cup, over hallel and nirtza, since he can still recognize to praise the divine and he is still "nirtza," desired by G-d.