The beginning of Act Four presents us with a picture of the witch hunt after its first opening few months when people are beginning to lose their wonder and shock at what has happened and are beginning to question the authority of this theocracy and the way that the witch trials are accusing so many righteous and good people of being witches. Although in Salem, it appears that Danforth's rule is still firm, Parris greatly fears that the hanging of Proctor and Rebecca Nurse on this day will be something that will stir revolt in the populace of Salem and cause them to rise up against Danforth and himself.
In particular, what has helped to spur his fears is the news that in Andover, where they were conducting similar witch trials, the people have stood up against the court and dismissed them, ending the witch trials and saying that they reject the court and its power. Note what Parris says to Danforth regarding this:
Andover have thrown out the court, they say, and will have no part in witchcraft. There be a faction here, feeding on that news, and I tell you true, sir, I fear there will be riot here.
The revolt in Andover thus coincides with a persistent doubting and questioning among the people of Salem as to the legitimacy of the courts and the claims that they make. Parris ultimately fears that this could result in danger for himself.