By the time Act Four begins, the people of Salem Village are nervous and worried about the continuation of the trials. Other neighboring villages have overthrown their courts or discontinuing their hearings.
Perhaps what is most unsettling is the fact that the people scheduled to hang that day-John Proctor, Rebecca Nurse, and Martha Corey- are well-respected in the village. Parris feels that his only hope of remaining in his position as minister in the village (one that is voted upon and not given automatically) is to show good faith. If he can help to "save" one of these respected citizens, he will ensure his survival for a bit longer.
He also states in Act IV, "Tonight, when I open my door to leave my house - a dagger clattered to the ground....There is danger for me." He senses the implicit threat behind the presence of the dagger and realizes that more than his position may be at stake; his life may be in danger.
Finally, Parris is also in possession of some vital and disturbing information, information that will cause even Danforth to question the entire process. Parris knows that Abigail has taken all of his money and run off to parts unknown.
Throughout the entire play, Parris has been a person who has been more concerned with the opinions of others than in his role as religious leader. Now, those opinions have turned against him and he knows that saving John Proctor is his only chance for salvation.