There really isn't enough evidence to be absolutely sure what the purpose of that conversation is in the beginning of Act IV but there are a few possibilities. First, both characters have confessed to witchcraft so it is possible that they are having this conversation because they know that Herrick is on his way into the jail and want to keep up their "act". The second possibility is that in the few months that they have been imprisioned, they have come to believe that they are really witches or that they have just been playing this role for so long that it has become second nature to them.
Parris has changed in that he now fears that the people of Salem are going to kill him since Abigail has disappeared and he has found a dagger stabbed into his door. He goes to the court and begs them to postpone the hangings of Proctor, Nurse, and Corey.
Hale returns to try to convince the people who he knows are innocent - Proctor, Corey, and Nurse - that they have to confess to witchcraft to save themselves because he now realizes that the girls were lying and that the entire situation should never have happened.
Hale has decided that the court was being used as a tool for vengeance and turns against it completely. He has spent time meditating and fasting to find the truth in his heart and soul. Parris is stricken with fear after Abigail robs his money and runs away, he wants the whole witch trial episode to end. He is afraid that he will lose his job or his life.
The court cannot alter its behavior just because Abigail and Mercy have run away, they have already executed 12 people. Danforth must continue to pursue the "truth as defined by the girls" because he fears retribution from the town, a revolt, a riot the town could put Danforth to death for his part in the trials.
Danforth wants Elizabeth to convince John to sign the confession. Elizabeth tells John she was partly responsible for the affair with Abigail, she was a cold wife. She respects him again, loves him. She doesn't take Hale's advice to convince John because she believes that he has saved his soul by refusing to be used by evil. Proctor gives up his life to save his immortal soul. He gets his dignity back, having confessed to his affair in public.
Proctor dies a decent, honest, Christian man, cleansed of his sin of adultery. He chooses to die a whole man, rather than live a life defined by a lie.
Corey was tortured, heavy stones were placed on his chest to encourage him to cooperate with the court. He refused, asking for more weight to be added, as a result he died during the torture. He is pressed to death for refusing to plead to the charge against him. He remained silent, preserving his land for his family by not pleading innocent or guilty to witchcraft.