Which four characters finally confront one another in Scene 1 of Act V of Julius Caesar?

Act V, Scene 1 opens just before the Battle of Philippi. On one side is the army of Octavius and Antony. On the other side are the armies of Brutus and Cassius. Since Shakespeare cannot show two armies fighting on his stage, he has these four men hold a "parley" prior to the engagement. Brutus is obviously hoping that they can come to some peaceable agreement without a bloody clash of two huge forces. He begins the parley by asking:

Words before blows. Is it so, countrymen?

If only he can get them talking, he might be able to talk them into a truce. He is reminding them that they are all fellow countrymen and asking, in effect, whether they insist on fighting. This is in character for Brutus. He is a man who believes in reason and justice rather than force. He does not, however, want to be the one who asks for a truce, since that would show that he feels weak.He is framing his words to get Antony and Octavius to give some indication of whether they would consider settling their differences peacefully, leaving it up to them. This is futile, but one wonders what exactly Brutus had in mind. Would he consider letting the triumvirate of Octavius, Antony and Lepidus retain the rulership of the Roman Empire while he retired to a country home somewhere and spent his time reading and meditating, which is all he likes to do anyway?

Octavius is a young hothead and wants to fight. But he probably would not insist on a battle if Antony indicated that he was willing to talk about a truce. Antony is in no mood to talk about peace because he is still furious about the assassination of his good friend Julius Caesar.

Villains! You did not so when your vile daggers
Hack'd one another in the sides of Caesar.
You show'd your teeth like apes, and fawn'd like hounds,
And bow'd like bondmen, kissing Caesar's feet,
Whilst damned Casca, like a cur, behind
Struck Caesar on the neck. O you flatterers!

Why does Shakespeare stage this parley when nothing comes of it? For one thing, the four men in armor have to represent all the soldiers who cannot be shown on a small stage. For another thing, it shows who is fighting and what they are fighting about. It also characterizes the four principals of the play. Antony is vindictive. Octavius is a hothead who wants to assert himself as Julius Caesar's heir. Brutus is a reasonable man but will fight if he has to. Cassius, as usual, is looking out for himself. He flatters Antony outrageously when he says:

The posture of your blows are yet unknown;
But for your words, they rob the Hybla bees,
And leave them honeyless.

Cassius is probably thinking that if they should lose the battle, he would like Antony to have a somewhat cordial feeling toward him because of his friendly attitude and his praise of Antony's funeral speech. At the same time, Cassius is taking a subtle dig at Brutus by reminding him that, in the first place, he didn't want Antony to be allowed to speak at all, and, in the second place, that Antony's speech was far more eloquent and effective than that of Brutus. By seeming to favor Antony over Brutus like this, Cassius may be dropping a subtle hint that he might be willing to go over to the side of Antony and Octavius against Brutus. That would mean certain disaster for Brutus; but Cassius, as usual, is thoroughly selfish, unscrupulous, and opportunistic. He doesn't expect to win this battle, and he warned Brutus against getting involved in it at this time and in this place.

Shakespeare's audience should enjoy this scene. They are seeing four great men all together for the only time in the play, and they are experiencing the illusion that they are witnessing a famous event in world history.