Act four takes place months after acts one through three have taken place. Act three ended with John Proctor's arrest. The text says that it is now fall, so all of summer has passed and the time frame is now most likely somewhere in October or November.
Act four itself takes place over a time period of just a few hours. Most of the act takes place within the jail where either Hale or Parris is attempting to convince the accused to confess to witchcraft. If they confess, they save their lives. Elizabeth is tasked to talk to John, and he decides to confess to the lie. Everybody is happy, but John recants his intentions, when the judges require him to sign his name on the confession. John can't bear saving his own life when the others stood faithfully in their true convictions. John is then taken away to be hanged.
I blacken all of them when this is nailed to the church the very day they hang for silence!