Brutus' internal conflict is definitely a part of the larger conflict of the fourth act, but is not as central as the conflict between the triumvirate and the conspiracy.
The big question of the fourth act is "Who will win the day?". Though the more interesting question, to me, is "How will Brutus deal with his sense of moral compromise?"
Act IV of Julius Caesar is rife with conflict. First, the triumvirate have conflict among themselves as they decide who shall live and who shall die. Then, the remainder of the act involves the personal and strategic conflicts of Brutus and Cassius. Before the troops Cassius claims that Brutus no longer loves him, but Brutus tells him not to display his emotions in front of the soldiers. Then, once inside the tent, they disagree on military strategy as Cassius feels the soldiers should remain in Sardis and force their enemy to come to them while Brutus disagrees, ordering the troops to march to Philippi. The demise of the troops and Brutus and Cassius are foreshadowing by the announcement of Portia's suicide which suggests the suicidal move of marching to Philippi.