Final answer:
In Act 4, Scene 3 of 'Julius Caesar', situational irony is present when the outcome of events contradicts the characters' expectations, such as the chaos following Caesar's assassination, which the conspirators had aimed to prevent by their actions.
Step-by-step explanation:
The situational irony in Act 4, Scene 3 of Shakespeare's Julius Caesar stems from the expectations of the audience and the characters within the scene. Situational irony occurs when the actual outcome of a situation is different from what was expected. In this specific context, the characters involved may have anticipated certain events or outcomes that are revealed to be quite different, thus creating an ironic situation. For instance, Brutus and Cassius, after assassinating Caesar to prevent the rise of a tyrant, find themselves in contentious power struggles that lead to civil war, reflecting the unintended consequences of their actions. This is ironic because their intention to preserve the Republic ultimately results in further chaos and a shift towards autocracy, contrary to their goals.