It seems there might be a misunderstanding in your question. "Julius Caesar" by William Shakespeare consists of five acts, not six. In Act 4, we see significant developments for the characters Brutus and Cassius as they grapple with the consequences of Julius Caesar's assassination and their subsequent actions.
In Act 4, particularly in scenes where Brutus and Cassius meet at Brutus's camp, tension arises between them due to disagreements over politics and their military strategies. They argue about issues of honor and leadership. Brutus accuses Cassius of corruption, while Cassius defends himself, claiming he has not done anything wrong. This confrontation highlights the strain in their alliance and foreshadows the challenges they will face as they prepare to engage in battle against the forces loyal to Caesar.
Once they reconcile, they resolve to unite against their common enemy, though the underlying tensions remain. If you meant to ask about a specific moment or theme within the play or another work, please clarify, and I'd be happy to help!