Act IV opens after Brutus and Cassius have fled from Rome. The first short scene focuses on Antony, who has taken control of Rome. He has allied himself with two men: Octavius, who is Caesar's nephew, and Lepidus, a respected soldier. Together, these three men are reviewing a list of men whom they believe should be killed to eliminate any potential threats. Antony agrees, without much reservation, that his nephew can be killed so long as Lepidus' brother will also be put to death. Antony also suggests that they try to manipulate Caesar's will, using the money Caesar had left to the Roman people for other purposes.

     Lepidus soon departs, and Antony and Octavius discuss him. Antony does not feel that Lepidus is a man who is fit to rule Rome, though Ocatvius suggests that Lepidus is indeed a "valiant soldier." Antony then goes on to compare Lepidus to a beast of burden, suggesting that they are merely using him as a tool as they take control of Rome.

     Antony then mentions Brutus and Cassius, telling Octavius that they have raised an army. Antony concludes they must build their own army and stop any uprising that Brutus and Cassius might cause.

     Scene II shifts to Brutus and Cassius. Brutus is camped out with his army, waiting for Cassius to arrive. Brutus worries that Cassius shows signs of a "hot friend cooling," or someone who is growing distant in their friendship. Cassius does arrive with an army, and the first thing he does is accuse Brutus of having done him wrong. Brutus cautions him that they should not argue in front of their soldiers, and they step into Brutus tent.

     Scene III shows the two men arguing in relative privacy. Each has grievances with the other. Cassius is angry because Brutus condemned a man who took bribes, despite the fact that Cassius told him not to. Brutus, ever concerned with honesty, is angry because he also believes that Cassius has himself been taking bribes. For Brutus, such immorality would completely defeat the purpose of having killed Julius Caesar; in his mind, he wonders why they would kill a corrupt man only to become corrupt themselves. Brutus is also angry because he sent to Cassius for money, and Cassius did not provide it to him.

     Their argument descends into name-calling, and Cassius finally offers Brutus his dagger, telling his friend that he may as well kill him. This breaks the tension, and Brutus tells Cassius they are both simply irritable. Cassius asks Brutus to forgive him, blaming his outburst on the bad temper he inherited from his mother. After a time, Brutus confesses that many things have been weighing heavily on his mind. Most notably, he has learned that his wife, Portia, whom he left behind in Rome, has committed suicide. Cassius wonders how Brutus didn't actually kill him with such a burden on his mind.

     Two men, Titinius and Messala, enter to report news to Brutus and Cassius. They report that Antony and Octavius have put a hundred senators to death in Rome. Messala also asks if Brutus knows anything about Portia, and Brutus pretends to know nothing about her. Messala then reports that she has died. Brutus plays it off like he is not too upset, likely so that his soldiers will not see him as a grief-stricken and thereby judgment-impaired leader.

     Cassius and Brutus then discuss their battle plans. Octavius and Antony are raising an army, and Brutus and Cassius know a fight is inevitable. They have basically two options. Brutus suggests they march toward Phillipi and meet their enemy halfway. He believes that, if they stay put and let Antony's army come to them, that they themselves will run out of supplies. Cassius, however, believes that they should stay put, arguing that Antony's army will tire itself with the long march. Ultimately, they decide to go with Brutus' plan of marching toward Phillipi.

     Cassius departs, and Brutus reads alone in his tent with his servant Lucius. When his servant falls asleep, a ghost enters the scene. It is the ghost of Caesar. The ghost tells Brutus that he will see Brutus again at Phillipi.

     Act IV is one of the shorter acts in the play, but it lays the groundwork for the battle that will occur in the last act of the play. It is also heavy in foreshadowing, or suggestions of what will happen later in the play. So far, the audience will likely be aware of Brutus' consistently poor decision making. First, he decided to join the conspiracy, which clearly didn't work in his favor. He also decided not only to let Antony live, but also to allow him to speak at Caesar's funeral. Because of Antony's clever manipulation of the Roman public, Brutus was forced to flee Rome. The audience will likely guess that Brutus' decision in terms of his battle plans also do not bode well.

     Additionally, the news about Portia in this scene shows that Brutus is, indeed, grief-stricken. This might suggest that, even though he pretends not to be bothered by her death, that perhaps all of his decisions are clouded by his grief. He seems doomed to fail.

     The scene is also revealing in terms of characters. The audience is given a glimpse a whole new side of Antony. He proves himself to be a shrewd and merciless leader now that he is in control of Rome. He has no qualms about putting people to death-even if they are family-nor does he mind using another man, like Lepidus, for his own purposes. He even goes so far as to manipulate Caesar's will. He is a clear contrast to Brutus, who is forever concerned with doing the right thing for the right reason.

     Finally, this scene also reveals a good deal about Cassius. If there was any doubt before about his corruption, it now becomes apparent that, unlike Brutus, he is not above corrupt acts like taking bribes. He is also shown to be the sort of person who has difficulty even accepting responsibility for his actions. For even something so small as an argument, he blames his unkind words on the bad temper he inherited from his mother, rather than simply offering Brutus an apology. Additionally, for the third time in the play, he offers of up own neck at the slightest sign of trouble, seemingly willing to take an easy out rather than have to deal with consequences. This will be important to understanding his character later in the play.