In The Crucible, why does Miller begin Act 4 in a jail cell?
The jail seems like the natural progression as far as setting. The first act is set where the hysteria begins: in Reverend Parris's house. The second act is set two weeks later, in the home of the protagonist, a site that shows the audience the speed with which the hysteria has spread because so many characters converge there: the Proctors, both soon to be accused; Mary Warren, accuser; Mr. Hale, witch hunter; Giles Corey and Francis Nurse, husbands of the recently accused; and Herrick and Cheever, come to arrest Elizabeth. The third act takes place in the courtroom, allowing us to see, firsthand, the corruption those legal proceedings. Where else could the fourth act take place but in the jail? Practically every character to whom we've been introduced is there: court officials, religious leaders, and the convicted. At the least, we must follow John Proctor -- he's been the one constant in each act, each place -- and he is in the jail now, awaiting death. Just outside the jail is the site of his redemption, when he finally comes to understand that he has not irretrievably lost his goodness and integrity, that he can still exercise his ethics without being a hypocrite. The audience must be present to see his realization in order for us to understand many of the themes of the play.
The beginning of this Act gives Miller the opportunity to demonstrate the results the hysteria has caused in a typical community. Sarah Good, a once well respected Christian woman, is now singing the praises of the Devil. Marshall Herrick is extremely drunk and offering the cell mates drinks. There are cows in the streets, crops rotting and homeless children wandering from home to home hoping that their current care-takers will not be accused of witchcraft. This jail setting also symbolizes the shackling power of the previous acts of the girls and the magistrates up to this point. It presents the idea of justice unfairly achieved. We as audience members get enraged that the people who are in the jails are there because they are all good people.