Cassius is angry that Brutus punished an officer for a small offense, even though he'd written to him asking that the man be pardoned. Brutus accuses Cassius of habitually getting people out of trouble in exchange for bribes, adding that since they murdered Caesar for his corruption, it would be hypocritical of them to be corrupt now.
Conflict between Brutus's morals and Cassius's pragmatism, which began with the disagreement about whether to kill Antony, comes to a head here. Brutus's adherence to his principles is beginning to impede their efforts.
Cassius is insulted, and says that he's an abler soldier than Brutus. Brutus disagrees, saying he is not afraid of Cassius. Cassius says that even Caesar never insulted him this way, and Brutus says that Cassius was too afraid of Caesar to give him reason. Brutus continues, saying that Cassius denied him money for his army, even though Cassius is richer than he, because of his corruption. Cassius says that Brutus is being unfair. Brutus accuses Cassius of loving flattery. Cassius makes a show of asking Brutus to kill him, if he really thinks him so dishonorable.
As they argue about Caesar, they begin to mirror him. Cassius's dramatic gesture of baring his chest and asking for death is the same one Caesar used before the crowd, and Brutus's refusal to repeal the officer's punishment is identical to the argument Caesar made just before his murder. Caesar was a successful politician because he combined elements of both Brutus and Cassius.
Brutus softens, and apologizes. Cassius apologizes too, saying that he inherited his temper from his mother. Brutus says that from now on, he will regard Cassius's mother as being responsible for his bad moods, not Cassius himself.
Esteem for masculinity is demonstrated again.
An old Poet enters the tent, arguing with Lucillius, who is trying to keep him out. The Poet rhymes badly, saying that Brutus and Cassius should not be arguing. Brutus and Cassius mock him and have him sent away.
Like the last comic scene with Cinna the poet, this brief interlude breaks tension before the focus changes. The original actor may have impersonated one of Shakespeare's rivals.
When they are alone, Cassius says that Brutus's recent anger was uncharacteristic of him. Brutus tells Cassius that Portia, afraid that Octavius and Antony will win, has committed suicide by swallowing hot coals. Cassius expresses sympathy, but Brutus says he doesn't want to talk about it any more. Lucius is ordered to bring wine, and Brutus and Cassius drink to their reconciliation.
Portia's suicide refreshes the audience's sympathy for Brutus, and helps explain the preceding argument, since losing his temper is so uncharacteristic of Brutus. Her manner of death is typically gruesome, as if intended as final proof of her unfeminine toughness.
Titinius and Messala enter with news from Rome. They say that Antony, Octavius, and Lepidus have executed many senators. After some hesitation, Messala tells Brutus of Portia's death, thinking he does not know yet. Brutus makes a show of acting unaffected, and Cassius commends him for his strength.
This is the only scene where Brutus "acts," exaggerating his stoicism in front of the troops. Such behavior would be second nature for Caesar, Antony, or Cassius.
Brutus suggests they march to Philippi to meet the triumvirate's army immediately. Cassius says they should let the enemy come to them instead, so that they'll be tired out. Brutus says this would give the enemy time to enlist more troops, whereas numbers would favor them if they attacked now. Cassius agrees and they say good-night. Cassius leaves with Titinius and Messala.
As always, Brutus wins the argument with Cassius—but here, it is because his suggestion is more pragmatic, rather than it being a case of pragmatism versus morality.
Brutus orders Lucius to play music and Varrus and Claudio to sleep in his tent, in case he should need to send a message to Cassius. Varrus, Claudio, and Lucius all fall asleep. The Ghost of Caesar appears, identifying himself as "Thy evil spirit, Brutus" (4.2.333) when, and saying he will appear again at Philippi, before vanishing. Brutus wakes the others, and sends word to Cassius to prepare to march.
The Ghost's manner of identifying himself implies that murdering Caesar was the only thing Brutus ever did wrong. The revelation that he will come again implies that Brutus will die at Philippi, but Brutus seems unaffected. He may realize here that killing Caesar was wrong.