In Act IV Scene 1 Antony, Octavius, and Lepidus, the ruling Roman triumvirate, are debating those to put on their "kill list." After Lepidus leaves, the audience and Octavius learn that Antony does not like Lepidus and thinks poorly of him.
In Act IV Scene 2, Brutus is visited by an angry Cassius while outside of his tent. Brutus tells Cassius that they must speak inside so there are no eavesdroppers
In Act IV Scene 3, Cassius complains to Brutus about how Brutus fired a soldier for taking bribes. Brutus accuses Cassius of doing the same, and they end up arguing. Cassius makes a dramatic spiel about how he wishes Antony would kill him so we would not have to endure Brutus being rude, and they apologize.
Brutus then confesses the reason he was so distraught and easily angered: Portia has died. The two mourn and decide when to fight the battle. After some negotiation, Cassius sides with Brutus to march to Philippi the next day.
Brutus calls in Lucius to play his instrument to help Brutus sleep. While playing, Lucius falls asleep. Brutus laughs and is pleased that Lucius tried so hard, and lets him rest.
As Brutus tries to sleep, he is haunted by the ghost of Caesar who tells him they shall meet again at Philippi. Instead of taking precautions, Brutus tells his soldiers to start moving earlier than anticipated in the morning.