At the end of Act III, Reverend Hale "denounces" the proceedings. This scene where Proctor attempts to provide multiple depositions to the court, one being Mary Warren admitting that there is no witchcraft happening, ends in Proctor himself being taken into custody for witchcraft. Hale realizes that the girls are lying, and he leaves. Act IV begins after several months have passed, in the fall. A conversation between Herrick and Danforth reveals that Hale has returned around midnight, and Danforth is suspicious of him. Herrick says, "He goes among them that will hang. . . he prays with them." Later, Hale speaks to Elizabeth and begs her to convince Proctor to confess so that he will be saved. Hale says to Elizabeth,
"Let you not mistake your duty as I mistook my own. I came into this village like a bridegroom to his beloved, bear gifts of high religion; the very crowns of holy law I brought, and I what I touched with my bright confidence, it died."
Later, when Proctor decides to hang instead of confess, Hale screams, "Woman, plead with him! It is pride, it is vanity!"
Hale feels incredibly guilty for his part as a major catalyst for the witchtrials. He is doing everything he can to prevent further bloodshed. Because of this shift, Hale can be classified as a dynamic character, one who changes throughout the course of the play.
Danforth is visibly upset when he learns that Rev. Hale has returned to Salem. He says he doesn't have the right to be here. He demands to know why he is back in Salem.
Rev. Hale returns to Salem to try and save as many of the accused as possible. He is really concerned for Rebecca Nurse. She has been locked up for three months and hasn't spoken a word in this time. Rev. Hale is with her, trying to get her to confess to witchcraft and find salvation. He is also there praying for all of the accused. The sun is getting ready to rise, and Rev. Hale tells Danforth, that he needs more time. Danforth refuses to give him anymore time. Danforth is adamant that the executions go as planned.
"Now hear me, and beguile yourselves no more. I will not receive a single plea for pardon or postponement. Them that will not confess will hang. Twelve are already executed; the names of these seven are given out, and the village expects to see them die this morning. Postponement now speaks a floundering on my part; reprieve or pardon must cast a doubt upon the guilt of them that died till now. While I speak God's law, I will not crack its voice with whimpering. If retaliation is your fear, know this-I should hang ten thousand that dared to rise against the law, and an ocean of salt tears could not melt the resolution of the statues. Now draw yourselves up like men and help me, as you are bound by Heaven to do. Have you spoken with them all, Mr. Hale?"
This quote by Danforth, shows that no matter what he is going on with the execution. It doesn't matter that these people are innocent, Danforth only cares about how it would look on him if he allowed the people to go free.
I assume you mean his return in Act IV. He came back to try and get the people who are about to be hanged to confess in order to save their lives. He tries to get Danforth to postpone the hangings as well. Hale now knows the trials were lies started by teenage girls who got caught doing something wrong. He is mostly concerned for Rebecca Nurse and John Proctor who are the first of the town's upstanding citizens to be hanged. He wants them to confess so they won't hang. He's trying to save their lives.