What is the dramatic importance of the concluding scenes of Act 2 and Act 4 in Arthur Miller's The Crucible?

So many plot arcs converge at the end of Act II that it's difficult to count them all. In this scene, John and Elizabeth are at home, discussing the happenings in Salem and their concern that they may be directly affected. Earlier in Act II there was an argument arising from the tension between them as they confront Elizabeth's suspicions about John's feelings for Abigail, and John's frustration that Elizabeth has not forgiven him. This emotional context provides a backdrop to the rest of the scene. We learn that Mary Warren has been manipulated by Abigail to place a poppet in Elizabeth's house to be used as evidence against her: this shows us the depth of deceit occurring among the girls with Abigail as their ringleader. We learn that John and Elizabeth now agree that Abigail is hell bent on nothing less than having Elizabeth jailed and hung for witchcraft, in order to make John available to her.

The arrival of Reverend Hale adds an additional level of tension, as he questions the Proctor's standing in the church. Soon after Francis Nurse arrives, to say his wife Rebecca has ben accused and arrested. Tensions rise at the realization that even a pious elderly woman like Rebecca could be accused of witchcraft. When Ezekiel Cheever arrives, it comes as no shock that he has a warrant for Elizabeth. Proctor vows to prove his wife's innocence, and his anger at the proceedings and disgust with Abigail are both palpable. The primary dramatic importance of this scene shows how the witch trials were based partly in personal antagonism and vendettas. We also see the significance of John and Elizabeth's relationship and how it underpins the entire plot.

The end of Act 4 is in a jail cell several months after Proctor and his wife are convicted and sentenced to hang, and Reverend Hale, who has denounced the court proceedings, comes to beg Elizabeth to reason with Proctor and get him to confess. He does, to save his life, but at the last moment he tears up the confessions, and Elizabeth supports his decision, knowing his integrity means more to him than his life. Their love is reaffirmed in this moment, again showing the dramatic importance of their relationship and its effect upon their motivations and actions.