what happens in the Brutus's tent

Brutus saddens Cassius with news of his wife's death and then makes military plans. Here are more specifics from the GradeSaver summary: Brutus sadly informs Cassius that his wife Portia is dead. She swallowed live embers after Antony and Octavius assumed power. When two underlings enter the tent, Brutus stops talking about Portia and focuses on the military matters at hand. In fact, when one of the men asks him about his wife, he denies having heard any news about her. Brutus convinces Cassius during the strategy meeting that it would be best for them to march to where Antony and Octavius are located in Philippi (near modern Greece) in order to defeat them before they get too strong, gaining additional soldiers on their march. Cassius reluctantly agrees to Brutus' plan and departs for the night.

Brutus calls some men into his tent in case he needs to send them away as messengers during the night. He makes them go to sleep. He himself stays up reading, but he is disturbed by the ghost of Julius Caesar who appears. The ghost tells Brutus that he is his "evil spirit" (4.2.353) and that he will be on the battlefield at Philippi. Brutus is so shaken by this image that he wakes up all the men in his tent and sends them to Cassius with orders that Cassius should depart before him the next morning.

Brutus and Cassius argue over right and wrong. Cassius feels betrayed by Brutus who will not condone greed and corruption. Brutus reminds Cassius of what happened to Caesar, and why it happened, that Caesar's died in the name of justice.

Cassius responds in anger; his countenance is threatening, but Brutus doesn't back down declaring that they aren't robbers, they don't collect bribes like lowly men without honor. He declares that he is not that kind of man, let alone that kind of leader, and he adds that Cassius is no longer the man he used to know. He orders Cassius out. Their friendship is lost.