Hamlet appears to become more aggressive and determined during the course of Act 4 in Shakespeare's Hamlet. Of course, this isn't the first time he has shown aggression or determination, so whether the act demonstrates actual change might be a matter of opinion.
Hamlet feels rebuked again in Act 4, this time by Fortinbras's army (Act 4.4). He concludes his thoughts with:
...Oh, from this time forth,
My thoughts be bloody, or be nothing worth! (4.4.64-65)
The army Hamlet sees is on its way to fight and die for a worthless piece of land. If these soldiers are willing to do this for almost nothing, Hamlet reasons, how terrible is it that he has a father to revenge, and yet has so far done nothing. His resolution strengthens and he is determined to carry out his revenge.
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)