The Crucible, by Arthur Miller, is a recounting of the Salem Witch Trials using many of the real characters and actions of the historical event. In this environment of suspicion and accusation, it should not be surprising to find anyone in prison in Salem.
In one sense, perhaps it is a surprise to discover that Tituba is in prison at the beginning of Act IV. While we know that Parris is intent on placing blame anywhere he can in order to make himself look good, Tituba "comes to Jesus" in the first act and Parris seems quite pleased with this. Parris has generally gotten everything he wanted, including John Proctor's comeuppance in the court and in the town, so there is little need for Parris to turn on Tituba for anything.
On the other hand, during her pre-conversion confession, Tituba expresses her hatred of Parris and he is stunned to hear it. He knows that Tituba was the lone adult with the girls in the forest the night all this trouble began--well, the only adult besides him--so it would be natural for a spiteful man like Parris to want to see her punished.
Tituba is in prison with Sarah Good, and neither of them seem to be quite in their right minds. While Tituba is not awaiting execution like Proctor and the others, perhaps she is here because she has become a babbling, frightening woman. The loss of her beloved Betty to the court and Abigail's mean treatment of her could have caused her to lose touch with reality. She talks about the devil, claiming he is "pleasure-man in Barbados, him be singin' and dancin' in Barbados." She begs the devil to take her home, a sign that she may have become too addled to live in Parris's house any longer, and prison might be the safest place for her to be under those circumstances.
Tituba is in prison, but she is not awaiting execution. Those awaiting execution include John Proctor, Rebecca Nurse, and Martha Corey. All people who named others were not executed; they were put in prison.
The mood is one of one of doom because of the constant superstition and increase in hysteria. Almost every person is accused or suspected by others in the town. The most holy and devout are the ones being executed, while the liars are saved. The most evil people are deemed the officers of the court and allowed to be in charge. Children and animals have no one to care for them; farms are ignored; and no one listens to reason. It is a depressing, hopeless time.
As for Parris, he supports no one; he never has. He only cares about himself and what will happen to him. The only support he ever gave Tituba was to treat her like a slave, threaten to beat her to death, and to put blame on her and not himself.