How can Act IV be considered as the well-structured act of the play?

Act IV marks the beginning of the play's resolution; we see an enormous amount of allegory meant to provide us with a thorough understanding of the action. My favorite of these comes from the Duchess when she uses the fable of the salmon and the dogfish to refute Ferdinand's earlier fable of reputation, love, and death, which was meant as a warning to her.

Unlike Ferdinand, the Duchess sees death as a leveler between social classes....... and Shakespeare uses the biblical symbol of the fish to capture the Christian mind.

I don't know if I would consider this the most well-structured act, but I do see the falling action as an irreplaceable introduction to the end of what is a very strong moral lesson.