Reverend Hale changes greatly from Act I to Act IV. He comes to Salem as an "expert" on witchcraft. This is quite ironic because he has never actually discovered a witch. He proved in his town that one woman was not a witch. When he arrives, he has numerous books and every expectation that he will find witches in Salem.
In Act II he questions the Proctors on their "Christian character" in order to determine if Elizabeth is a witch, as Abigail has suggested. This is a man who believes. He does nothing as Elizabeth is arrested.
In Act III, he comes to believe John Proctor about Abigail. He does not believe that either Proctor is a witch and perceives Abigail to be false. He wants the court to allow Proctor a real lawyer, so that his allegations about Abigail will be presented in a correct manner. He believes Proctor and denounces the court at the end of the Act.
In Act IV, he returns to Salem to council all people awaiting execution to lie. He is trying to keep them from death and this is the only way. He does not believe that any of them are witches and is trying to save them in the only way that he can. He is a desperate man and uses desperate measures. He does not believe at all, any more.