In addition to affecting the mood and conveying the extent of damage even to those who were lucky enough to live, the first scene of Act IV also shows the hypocrisy of the trials and judges. Tituba and Sarah Good keep talking about when the devil is supposed to show up, what he is going to bring for them, and where he will take them. Just a few months ago, statements like these would have been absolutely believed, taken as fact, by Deputy Governor Danforth and Reverend Hale and Mrs. Putnam and most of the village, just as Tituba's and Sarah's original fabricated confessions were.
Now, however, when they tell these stories, they are considered to be insane and ridiculous, and this is how their previous "confessions" should have been viewed. The people and judges of Salem have developed a more accurate perception since then, and this change should mean that no more people are hanged as a result of stories like these. However, several more people are to be hanged today, and this emphasizes the terrible, hypocritical priorities of Danforth and Hathorne especially.
By opening Act IV with its chilling, yet pitiful scene of madness among the prisoners, as seen especially in Tituba's ravings, Miller powerfully conveys the depth of the misery, suffering, and insanity wrought by the witchcraft trials. This scene also creates a dark atmosphere and a somber tone for the tragic final events that are about to unfold.