What is the significance of the scene between Herrick and the accused witches?
In this scene, we are given clues about the attitude of the authorities in the wake of the famous witch trials. Herrick works as the marshal of the jail, in charge of checking on the prisoners. In this scene, he is pragmatic but kind in his actions; when Sarah Good asks for a sip of cider from his flask, he gives it to her. Sarah seems somewhat delusional, thinking their "Master" has arrived, but Tituba remarks that it just looks like the marshal. When Tituba says they are waiting for the Devil to take them away with his feathers and wings, Herrick agrees that he'd like to go, too:
Tituba: I’ll speak to him for you, if you desires to come along, Marshal.
Herrick: I’d not refuse it, Tituba; it’s the proper morning to fly into Hell.
In saying this, Herrick may be suggesting that he thinks the current situation is absurd, and, similar to John Proctor in earlier scenes, he thinks that the entirety of Salem Village had gone insane and has been taken over by the superstitious and vicious accusations of witchcraft. He seems to be willing to "play along" with Tituba, who at first appears to be very lucid, but then gives in to the same delusional chatter that Sarah is engaging in. So he decides to humor the women, but only to a point. Herrick is merely doing his job here, but he does seem to have sympathy for Sarah Good and Tituba.
When Tituba hears a cow lowing in the distance and thinks it must be the devil, Herrick says, "That's not Satan, just a poor old cow with a hatful of milk." He is not willing to continue to entertain her fantasy of being saved by the devil, and continues to hurry both women out of the jail cell to a different section of the prison. There is a sense that these are not the only accused witches Herrick has to deal with today, that this has become the recent reality of his job, and that he is coping as best he can by showing some amount of decency towards them.
There are a few possible explanations for the scene that takes place between Herrick, Goody Good, and Tituba at the beginning of Act IV. The first possible explanation is that the two women are obviously not witches and must put on an act for the jailer, Herrick. Another possibilty is that the women are slowly going insane from being in such horrible conditions for such a long period of time. At this point, Tituba and Goody have been in jail the longest, probably a few months, and might even believe that they might be witches at this point. Finally, Miller could have included this scene for comic relief. This is almost a comical scene; however, it follows an act that was full of drama and emotion and precedes a scene that will contain even more drama and emotion.