Antony and Octavius vs Brutus and Cassius
In Act IV, how does the relationship between Antony and Octavius compare to and contrast with that of Cassius and Brutus?
The two relationships are alike in that they are both based on mutual need. Antony and Octavius need to work together to defeat Cassius and Brutus; Cassius and Brutus have no chance to prevail against Antony and Cassius if they don't settle their differences and stay united in their war against their enemies.
The relationnships are alike also in that each one exhibits a leader and a follower. Antony's word takes precedence over that of Octavius. Cassius bows to Brutus's will even when Cassius thinks Brutus is wrong.
How are these relationships different? The best response here, I think, has already been mentioned. Brutus and Cassius do have a personal relationship whereas Antony and Octavius do not. Brutus and Cassius have been through a horrendous experience together. Together they have plotted and and acted to murder Caesar. Together they have been driven from Rome, running for their lives.
Because of these facts, there is another difference that is understandable, considering human nature. The relationship between Cassius and Brutus is much more emotional. They argue violently, and the argument gets personal.
The relationship between Antony and Octavius is cool and professional. For example, when Antony and Octavius have a difference of opinion as to cutting Lepidus out in dividing the spoils of war, Octavius lets Antony have the last word. Octavius raises some objections, but does not argue with him.
Antony and Octavius are more formal with one another--not truly friends. They are put together in the name of vengeance--Antony's friend and Octavius' relative Julius Caesar had been murdered. In fact, the connection between the three--Antony, Octavius, and Lepidus is strained. The first two do not recognize Lepidus as anything more than one to run errands and do their bidding, but they also seem to question where they stand with one another. Antony was Julius' right-hand man, but Octavius is a blood relative and seemingly the right choice for the "throne" had this been a dynasty and not a republic. There are issues of mistrust and tension between these two.
On the other hand, Brutus and Cassius are truly friends. They respect one another and treat each other as brothers. Aside from the spat they have over accepting bribes and engaging in dishonorable behavior, they are bonded in their act for the betterment of Rome. Brutus is an honorable man who believes all the way to his deathbed that he did what he did for Rome, not for personal advancement. Cassius, perhaps, was in it more for personal reasons...he admits through his actions and thoughts in the play that he was jealous of Julius Caesar. Whatever the reasons for each man participating in the murder of Caesar, it does not take away from the depth of their friendship.