What is the significance of the behavior of Tituba and Sarah Good at the start of Act IV?
I think that the significance of the behavior of Tituba and Sarah Good is extremely important in the Fourth Act. On one hand, their madness mirrors Parris', something to be revealed later on in the act. It is significant that those who are condemned and those who did the condemning are suffering the same type of fate. This helps to bring out the idea that the sense of evil and malevolence that sentenced people like Tituba and Sarah Good are coming back to haunt everyone. The "demonization" that started the narrative has now effectively spread to everyone, and the madness that both prisoners are experiencing as well as significant character who initiated the process is highly significant.
At the same time, the ravings of both women might also be a statement about the narrow and pedantic setting that drove them and everyone else insane. Tituba and Sarah Good are explicating the complex nature of the devil in their rantings. This is something denied and negated in the simplistically reductive atmosphere of Salem. Their articulations are bringing to light that the pursuit of that which is narrowly defined can have difficult results on those who have to live through such conditions. In this, the behavior of both women is as much a statement about Salem of the time period as well as their own predicament within it.
In addition, the ravings of Tituba and Sarah Good present two tragic ironies of the Salem witch trials in this work. Months earlier, in court, people who professed to see the kinds of things these women claim to have seen were believed without question. People condemned their friends and neighbors on the basis of just this kind of information. Now, however, instead of being listened to, Tituba and Sarah sit and rot in a cell, derided even by Marshall Herrick, who wryly wishes them a "happy voyage" to Hell.
Further, Tituba and Sarah both used to be good women -- Tituba, certainly, seemed pious and God-fearing in Act I -- and now they both long to be saved from Salem by the Devil, implying that the company of Satan has become preferable to the cruel treatment they have received by the townspeople. The witch trials, then, have actually driven innocent people to seek the Devil as a means of escaping their persecution in the town.