Which event Occurred in the nearly two-year time span between act 3 and act4 of The Tragedy of Julius Caesar?

The correct answer is:

Mark Antony and Octavius raised an army against Brutus and Cassius.

Between Act 3 and Act 4 of The Tragedy of Julius Caesar there were almost 2 year differente. After Julius Caesar died, Octavius, Lepidus and Mark Antony allied themselves and recruited an army against Brutus and Cassius.