Summary: ... nothing happened, really.
Antony, Octavius and Lepidus chat about senators they'll kill. Lepidus leaves to steal some money from Caesar's will. Antony and Octavius chat about how Lepidus is a pawn. They decide to work together to fight Brutus and co.
Brutus, conspirators, and soldiers chat about how Cassius did something wrong and Lucilius is not too happy. Cassius shows up with his army. Brutus asks to talk to him privately because their armies think they're good buddies. Cassius agrees.
Apparently, Cassius permitted bribes from Sardians (sardines?) to a Lucius guy. Brutus tells him off. Cassius is offended. Brutus points out they killed Caesar for justice. Cassius points out he is a better soldier. Brutus says he is not.
They argue for a few more pages, Cassius offers to let Brutus stab him, then they make up. A concerned poet shows up. I have no idea why he exists, except to demonstrate that the people think Brutus and Cassius are BFFs. Although Shakespeare is ordinarily a fan of tell more than show.
We suddenly find out Portia, Brutus' wife, committed suicide because Octavius and Antony were becoming more powerful. General tip: if you just killed the well-respected ruler and have been chased out of your city, bring your family along. Even if they don't commit suicide, the enemy could use them as hostages.
They drink wine. A conspirator called Messala comes in, and they chat about how Octavius, Antony and Lepidus executed some senators. Then they chat about battle plans at Philippi. Everyone goes off to sleep.
Brutus tosses around for a bit while his servant sings him to sleep. Caesar's ghost shows up. Unlike Macbeth, Brutus pretty much keeps his cool. "Why I will see you at Philippi then." All chirpy, no problem. And no one else saw the ghost, but I guess we knew that.
Next up: Act 5. Brutus commits suicide, and Antony takes over. Sorry, did I spoil that for you?