In Shakespeare's Tragedy of Hamlet, Hamlet had been quite indecisive in Acts I and II. In Act III, he kills for the first time (Polonius). This is important, even though it is by mistake. As a result, in Act IV, Hamlet is more focused on his role as avenger: he has killed and can kill again. Indecision is a memory.
In Acts I-III, Hamlet was unsure of who was who: Was his father's ghost telling the truth? Did Claudius really kill his father? Was his mother in on the murder? Could he really kill his uncle? Now, in Act IV, Hamlet knows that Claudius is a villain; therefore, Hamlet can anticipate his moves better.
In Act IV, Hamlet willingly goes to England, even though he is meant to be killed there. Knowing this, Hamlet plays both a verbal and physical game of cat and mouse. Upon his return, he will have the upper hand: he will be the cat. As a result, his use of verbal irony (sarcasm) increases giddily:
My mother: father and mother is man and wife; man and wife is one flesh; and so, my mother. Come, for England!
Hamlet is so giddy, he even plays hide-and-seek with Polonius' body. He says to Claudius:
Hide fox, and all after. (an old signal cry for the game of hide-and-seek)