In Act 4 of The Crucible, what happened in Andover?

When the witch trials began, those first accused were powerless people who occupied the lowest level in their Puritan society. They were not recognized as being worthy or respectable, which contributed strongly in their being singled out for prosecution. As the trials continued, however, the accusations reached higher and higher into Puritan society; suddenly, no one was safe from arrest and punishment. When the nature of those accused began to change, the people became less enthusiastic as general fear and discontent with the proceedings developed.

In the Puritan village of Andover, witch trials were being conducted simultaneously with the trials in Salem. In Act IV, word reaches the court in Salem that the people of Andover have risen up and thrown out the court in their village, putting an end to the prosecution and persecution. This news is especially disturbing to the court in Salem where the hangings continue, now with the most respectable and upstanding people of Salem, such as John Proctor and Rebecca Nurse, scheduled to die. The judges fear the rebellion in Andover will spread to Salem, which makes it imperative to get a confession from one of the respected citizens about to die. A confession from John Proctor, in particular, would serve to legitimize the court's proceedings.

In Act 4 of The Crucible, what event in Andover might impact the Salem trials?

In Andover, as in Salem, many have been accused of witchcraft. Evidently, according to Parris and Hale, the hysteria has grown there as it has in Salem. The authorities in Andover must be as stubborn as those in Salem, if not more so. Parris informs Danforth that there is talk of the people of Andover rebelling against the courts and religious authorities. They are fed up with the accusations, the oppressive condemnations of the authorities, and witchcraft itself. Parris says there is a group (faction) in Salem who, upon hearing of the rebellion in Andover, are considering a rebellion in Salem as well. "There be a faction here, feeding on that news, and I tell you true, sir, I fear there will be riot here." However, Danforth denies that there even is a rebellion in Andover. And Hathorne scoffs at the idea that people would be against executions. They truly show their stubbornness here. 

Shortly after Parris's warnings of rebellion and riot, Hale confirms them. He pleads with Danforth to at least postpone the executions to show some mercy. Otherwise, he says rebellion is a foregone conclusion: 

Excellency, there are orphans wandering from house to house; abandoned cattle bellow on the highroads, the stink of rotting crops hangs everywhere, and no man knows when the harlots’ cry will end his life - and you wonder yet if rebellion’s spoke? Better you should marvel how they do not burn your province!