Act Four, Scene One
At the rebel camp near Shrewsbury, a messenger brings Hotspur news that his father is sick. This bodes poorly for the rebels, since they need the full support of Northumberland to maintain their military force. Additionally, Northumberland is needed to help lead the troops into battle, since his persona is well known and provides inspiration to the men.
Hotspur's cousin Vernon arrives with news that the king and young Hal are leading armies against the gathered forces. Hotspur is eager to meet Hal in battle, and comments that when they meet only one of them will survive. However, Vernon has even worse news in that Glyndwr is unable to assemble his army as fast as needed, and will be unlikely to join the rebels at Shrewsbury.
Act Four, Scene Two
Falstaff has assembled an array of troops who are mostly incompetent. In order to earn money for himself, he conscripted young lovers and men of some wealth who did not want to fight, and who were therefore willing to pay him a fee for being released from duty. The remainder of his infantry is mostly made up of former prisoners and other poor men.
Hal arrives, and wonders at the "pitiful rascals" (4.2.57). He tells Falstaff to hurry up and meet the army at Shrewsbury, where the king has already set up camp. Falstaff comments that he will arrive at the end of the fray, yet at the beginning of the feast, implying he will reap the spoils of the war for himself.
Act Four, Scene Three
Hotspur and Douglas are arguing with Vernon and Worcester about when to lead the battle charge. They want to attack that night already, but Vernon counsels them to wait until the next morning for reinforcements to arrive.
Sir Walter Blunt arrives as a messenger from King Henry. He begs the rebels to tell him what their grievance is, and says the king will make amends to them if their complaints are valid. Hotspur tells Blunt that Bolingbroke had only returned to England to reclaim his title to Lancaster, not to overthrow Richard II. Hotspur claims to recognize the claim to the throne made by the Earl of March. Blunt asks if that is the message he should take to Henry IV. Hotspur instead tells Blunt that his uncle Worcester will visit the king the next day, and that he will withdraw for a short while.
Act Four, Scene Four
The Archbishop of York is afraid that Hotspur will be unable to defeat King Henry. Since he is a part of the rebellion, the Archbishop is scared that Henry will come after him as soon as Hotspur loses. To protect himself, the Archbishop sends Sir Michael with letters to his close friends and noblemen, begging them for military support.
The transition of Hal from a young madcap to a warrior prince is completed in this act. Vernon remarks to Hotspur that, "I saw young Harry with his beaver on / ...witch the world with noble horsemanship" (4.1.105,111). This marks the final break with his past for Hal, who now demonstrates horsemanship and military competence.
This is further evidenced by the disdain shown towards Falstaff when Hal encounters his mangy troops. He orders Falstaff to mobilize towards Shrewsbury at a faster rate, and remarks on the amount of money that Falstaff has stolen as a commander. No longer is Hal content to play the games that Falstaff enjoys, since the reality of the war is that men are going to be killed.