Final answer:
The tension between Brutus and Cassius in Act 4 of Julius Caesar arises from their differing perspectives on the conspiracy and political situation.
Step-by-step explanation:
The tension between Brutus and Cassius in Act 4 of Shakespeare's play Julius Caesar arises due to their conflicting perspectives on the conspiracy to assassinate Caesar and the subsequent political situation. Brutus believes that their actions were justified for the greater good of Rome, while Cassius starts to doubt their cause and becomes increasingly resentful of Brutus' decisions. This tension escalates as they argue over strategies and priorities, leading to a breakdown in their friendship and alliance.