In Act Four of The Crucible, Proctor decides that he will falsely confess to witchcraft in order to protect the integrity of his family and to avoid being made into a martyr. However, when he is asked to testify that others have participated in witchcraft as well, Proctor refuses to do so on principle; although he does sign a confession, he ends up ripping it up in order to avoid tarnishing the reputation of his family. Thus, Proctor chooses to accept his death sentence.
This is a stunning transformation of character, given the fact that Proctor begins the play as an adulterer who has lied to his wife, violated his marriage vows, and put his standing within the community at stake. Thus, what Proctor values at the conclusion of the play is the truth. As Elizabeth states before the curtain falls, "He has his goodness now." Proctor has re-located his sense of integrity and his personal moral compass.
At the end of "The Crucible," what John Proctor appears to value more than anything is his own integrity. He decides that his integrity is more important to him than his life.
At first, in this act, he had decided he was going to confess to witchcraft so he wouldn't be executed and wouldn't leave his wife a widow and leave their unborn child without a father.
But finally, after talking to his wife, Elizabeth, and after seeing Rebecca Nurse go to be executed without trying to get out of it, he decides that it's more important to be honest.