Final answer:
In Act IV of Shakespeare's 'Julius Caesar', Brutus and Cassius's friendship deteriorates due to political stress, ethical disagreements, and Brutus’s personal grief over his wife's death.
Step-by-step explanation:
The friendship between Brutus and Cassius changes in Act IV of Shakespeare's Julius Caesar primarily due to the stress of their political and military situation after the assassination of Caesar.
Their once strong mutually supportive relationship is strained by disagreements over leadership and ethical issues, such as the corruption Cassius demonstrates through his dishonest fundraising activities and Brutus’s uncompromising stand on morality and virtue. Additionally, the personal grief Brutus faces after the death of his wife, Portia, contributes to the tension between them, leaving Brutus emotionally vulnerable and further complicating their interactions.