Tituba opens Act 4 with Sarah Good and Herrick. Apparently, she has been sharing a cell with Sarah Good during her imprisonment.
I believe Miller reintroduces Tituba in Act 4 for several reasons. First, it allows the reader to see that Tituba has not been executed as others who were accused but refused to confess to witchcraft. Regarding this fact, Tituba not only confessed to witchcraft in Act 1 (to save herself), but now she also portrays someone who has fully embraced the Devil. Miller's dialogue for Tituba illustrates a person who has lost her sanity because of her confinement, very poor treatment, and her cell mate.
Most importantly, the playwright uses Tituba in Act 4 to stress how far the court and town have strayed from their original "pure" or religious goals. The world has been turned on its end and evil now appears good while good (in the Puritan sense) has engendered evil (the execution of innocent townspeople, the ruination of a town, and the raising up of the truly guilty). Tituba describes this phenomenon near the beginning of Act 4 by stating,
"Oh, it be no Hell in Barbados. Devil, him be pleasureman in Barbados, him be singin' and dancin' in Barbados. It's you folks--you riles him up 'round here; it be too cold 'round here for that Old Boy. He freeze his soul in Massachusetts, but in Barbados he just as sweet and--"
Clearly, through Tituba's words, Miller seeks to show that even the devil would flee a place like Salem--quite a strong condemnation.
In real life, I do not believe Tituba lost her sanity. When she finished her unjust jail term, she was releases and forced to work the rest of her life to pay off the debt she had supposedly accrued during her prison sentence.