The fourth act takes place in a Salem jail cell later in the fall. Marshal Herrick enters with a lantern, nearly drunk, and wakes up Sarah Good. Tituba is also in the cell. She says that they will be going to Barbados as soon as the Devil arrives. Hopkins, a guard, tells them that the Deputy Governor has arrived. Danforth discusses with Hathorne whether it is wise to allow the increasingly mad-looking Parris to spend so much time with the prisoners. Cheever remarks on the many cows wandering the streets, now that their masters are in jail. Hale has been begging Rebecca Nurse to admit to witchcraft.
Parris arrives and tells Danforth that Abigail has vanished with Mercy Lewis. They have taken Parris' strongbox and he is now penniless. Parris claims that there are rumors of a rebellion against the witchcraft proceedings in Andover. Hathorne reminds Parris that all have been happy with the Salem executions, but Parris reminds him that Rebecca Nurse and John Proctor are respected members of the community and their executions will not be taken as well. Parris suggests postponing these hangings, and admits that there seems to be dissatisfaction, as shown by the low turnout at Proctor's excommunication.
Parris worries for his safety, having found a dagger at his doorway. Danforth refuses postponement, as it would show weakness on his part. Danforth summons Elizabeth Proctor. Hale tells Elizabeth that he does not want Proctor to die, as he would then consider himself a murderer. He tells Elizabeth that God damns a liar less than a person who throws one's life away. Elizabeth claims that this is a devil's argument, but Hale says that we are not capable of reading God's will. Danforth wonders if there is any wifely tenderness in Elizabeth. Elizabeth asks to speak with her husband. Herrick brings in Proctor, who is now bearded and filthy. Proctor asks about Elizabeth's unborn child and the boys, who are kept by Rebecca's son Samuel. Elizabeth tells Proctor that Giles is dead; he would not answer to his indictment and the court pressed him to death, laying stones on his chest until he pleaded aye or nay. His last words were "more weight."
Proctor asks Elizabeth what she would think if he confessed, but Elizabeth says that she cannot judge him. She says that she will have him do what he wishes, but she does want him alive. Proctor says that he cannot mount the gibbet as a saint, as he is not a saint like Goody Nurse. Elizabeth says that she has her own sins to account for, and blames herself for forcing her husband to turn to lechery. Proctor states to Hathorne that he will confess himself, but he asks Elizabeth once again if it is evil. She answers that she cannot judge, but he asks in return who will judge him. When they demand a written confession, Proctor asks why he must sign. Danforth says it is for the good instruction of the village.
The guards bring in Rebecca Nurse, who is astonished that John is confessing. Proctor refuses to say that he saw Rebecca Nurse in the Devil's company, or anybody else. Danforth demands that Proctor prove the purity of his soul by accusing others, but Hale advises that it is enough that he confess himself. Parris agrees, but Danforth once again demands that Proctor sign the document. Proctor says that he has confessed to God, and that is enough. He asks Danforth whether a good penitence must be public. Proctor asks how he can teach his children to walk like men when he has sold his friends. Proctor wishes to keep only his name, and Danforth thus refuses to accept his confession. Danforth orders Proctor to be hanged. Hale begs Elizabeth to plead with Proctor to sign a confession, but Elizabeth states that Proctor has his goodness now, and God forbid she take it from him.
The fourth act of The Crucible largely concerns the perversion of justice that has occurred in Salem. Miller demonstrates this immediately in the comic interlude that opens the act. Tituba and Sarah Good are foolish comic foils whose claims of communing with Satan are intended to be absurd. Yet while these women are spared the gallows because they have confessed to witchcraft, those like Rebecca Nurse who refuse to admit to a crime they did not commit remain sentenced to execution. This large-scale inversion of justice is reflected in the larger workings of Salem society. As Parris claims, there is the possibility of rebellion because of the witchcraft trials, while the numerous people who remain in jail have caused the village to fall into shambles. This is yet another example of the irony of the witchcraft trials: while they meant to preserve the order of society, the trials throw Salem into a state of anarchy and rebellion.
However, since the previous act there has been a shift in the public opinion concerning the trials. Miller indicates that the citizens of Salem supported the trials when the victims were obviously disreputable members of the community, but the executions of respected figures like Goody Nurse are much more controversial. This reinforces the idea that the Salem witch trials were in part vindictive; the purpose of the trials was not to remove witches from Salem, but rather to remove certain members of the community for other reasons. For the citizens of Salem, the executions only become unacceptable when they involve those honored members of the community, even if the charges against them have the same proof, or lack thereof, as those against the disreputable Bridget Bishop or Sarah Osburn. The implications of this are wholly cynical: the shift in public opinion is not a turn toward justice but rather an expression of personal preference.
If there is a sense of justice in The Crucible, it is meted out to Reverend Parris and Abigail Williams in this act. Reverend Parris reveals himself to be a fool capable of being easily manipulated by Abigail Williams, whose guilt seems obvious thanks to her sudden escape from town and theft of Parris' savings. However, even with these revelations casting further doubt on the validity of Abigail's charges, the Salem court continues with the trials and executions. The trials have taken on a life of their own, separate from the accusations of the principals, who set legal machinations in motion that even they cannot stop. This fulfills the theme of snowballing accusations that Miller established early in the play. The accusations began with Abigail Williams, but now, supported by the weight of the judiciary, the prosecution does not stop with her downfall.
Contrasting considerations of self-interest lead Danforth and Parris to beg John Proctor to confess to witchcraft. While Parris fears for his physical safety, Deputy Governor Danforth operates to defend the court from further attack. The change in Danforth's overt motivation is important. Previously, Danforth meant to uphold the integrity of the court, but here he suggests corruption to simply preserve the political stature of the government. Indeed, he even worries that postponing the executions would show the court's weakness. By prompting Proctor to give an obviously false confession, Danforth indicates that he likely believes that the witchcraft allegations are false. This fully demonstrates how the witch hunts have gained a life of their own; considerations of reputation and the political dynamic lead the court to continue with prosecutions and executions even when the original proponents of the trials are proven disreputable, and even when the political officials who run these trials show serious doubt in the validity of the charges.
The final passages of The Crucible concern ideas of martyrdom and justice. Miller places three of the accused as possible martyrs, each representing different methods and approaches to self-sacrifice. Giles Corey, the first of the noble victims of the trials, remains the comic tragedian even in the throes of his death. He does not passively accept the decision of the court, but struggles against the court's charges. Even when Giles Corey dies at the hands of the court, he chooses the mode of execution that will allow his sons to still inherit his property. In contrast, Rebecca Nurse accepts her fate passively, a long-suffering martyr to the court's injustice. Unlike the truculent Giles Corey, Rebecca Nurse only displays those most Christian qualities of resignation and turning the other cheek.
The critical test for John Proctor in this act is whether he will accept the martyrdom of Giles Corey and Rebecca Nurse or choose self-interest. Proctor himself proposes the question of whether a sinful man may accept martyrdom by clinging to principles he has not always upheld. The saintly Rebecca Nurse may accept martyrdom because it suits her character, but the sinful Proctor questions whether or not it is hypocrisy to stand for his principles when he is an overt sinner. Miller implies that Proctor may choose self-sacrifice because it is not a question simply of his reputation, but that of his family and his community. Proctor may not be an exemplar in all matters, but he could not serve as a father to his children if he were to so readily give up his name to preserve himself.
The second question of this act is whether it is a worse sin to lie to save oneself or to allow oneself to die. This is the fulfillment of the theme of self-preservation that has recurred throughout the novel. While Hale says that God damns a liar less than a person who throws his life away, Elizabeth calls this the devil's argument. Miller seems to support Elizabeth's position, for it is by giving self-preserving lies that Tituba and Sarah Good perpetuated the witch hunts.
Elizabeth Proctor serves as the moral conscience in this act of The Crucible. It is she who puts forth the most prominent arguments for Proctor accepting his own death, despite her stated wish that she wants her husband to remain alive. This could be interpreted as another manifestation of Elizabeth's cold nature, for she remains seemingly more concerned about abstract moral principles than her husband's life; Danforth even questions whether Elizabeth has any tenderness for her husband at all. Elizabeth is not to be played as a cold character, however. She refuses to influence her husband's decision despite her own wishes – he has earned her respect as a free moral agent, and she loves him all the more for his ability to make the right decision on his own.
The negotiations between Proctor and Danforth concerning his confession illustrate the theme of public versus private redemption. Proctor insists that his penitence remain private, while Danforth requires a public declaration of guilt and a further condemnation of other witches. It is this critical factor that allows Proctor to accept his martyrdom when he chooses to sacrifice himself to stop the perpetuation of the witchcraft accusations. Proctor thus answers his own concern about martyrdom, ending his life with an action that remains indisputably noble dispute the sins he has previously committed. He dies with his own name intact because, unlike so many others in front of the Salem court and the House Un-American Activities Committee, he refused to name names.