Act 4: What is the effect of Sarah Good's and Tituba's talk about flying south? Why does Miller include this talk?

We might point to several meanings behind the section of the play in question.

First, this conversation takes place at the very opening of the act (Act 4) and serves to set a particular tone of paranoia. Both Tituba and Sarah Good are frantic in their belief that the devil is coming to fly them away to Barbados. Sarah Good's description of the impending event in speaking to the Marshal Herrick is "Oh, it be a grand transformation, Marshal!"

Her exuberance fails to convince Herrick, who sees Sarah Good as a deranged person. This point of view is important in context. The town has recently sentenced many people to hang, including John Proctor, based on the word of people deemed credible.

Those "credible" people (i.e., Abigail) say things about witches and devils very similar to Sarah Good and Tituba, however, those people are believed because they are not outcasts or liminal figures as Tituba and Sarah Good happen to be.

Goody Good is a ragged and crazy woman who seems to live on the edges of town life.

Thus social position is equated to credence or "believability" in this passage of dialogue between Sarah Good, Tituba and Marshal Herrick. Seen in this light, we can perhaps re-evaluate the reasons for John Proctor's legal (and social) condemnation. He too exists as a liminal figure in the Salem community and his word is not believed. 

So here we have looked at two ways this passage functions in the text. It establishes a tone of paranoia that will be continued in Act 4 (especially with Parris) and it offers an insight into one of the play's central themes - "social truth" often conquers factual truth.