How does Andover differ from Salem?
The residents of Andover seem to have grown wise to the corruption of the court in its dealings with accused witches before the residents of Salem do. When Parris tells Danforth that Abigail has robbed him of his life's savings and fled the town, he fears that this fact in addition to the "news of Andover" will prompt a rebellion in Salem. However, Danforth tries to shut down his fears, saying, "Andover is remedied. The court returns there on Friday, and will resume examinations." Parris persists, insisting that the rumor is that there is a rebellion in Andover, that its residents have "thrown out the court" and will no longer hear cases against accused witches; worse for him, this rumor is fueling a "faction" in Salem. He knows that the town is dissatisfied already -- he's been physically threatened with a knife in his door, relatively few people came to Proctor's excommunication from the church, animals and orphans wander the streets while crops rot in the field -- and he fears that the hangings scheduled for that morning will prompt a similar rebellion. Thus, Andover seems a few steps ahead of Salem.
It's not clear exactly how the towns differ; however, Andover's citizens did band together to throw out the court. They apparently had enough of the accusations and hangings. It appears as Act IV progresses, that Salem's citizens will do the same. In fact, that is one reason Danforth wants Proctor to confess. If he simply sets him free, it will prove that the court's findings were unjust. This would give the townspeople, who are already growing tired of the proceedings, the initiative to overthrow the court.