The answer is indeed the fourth option: Brutus says that he is about to do something that will make him well, but ends up killing himself.
We may define situational irony as a situation in which a certain outcome is expected to happen, but the complete opposite ends up taking place. A simple example would be a marriage counselor asking for a divorce. No one would expect a person who advises couples and saves marriages to end his/her own marriage.
In literature, situational ironies come as a surprise to readers, having them reflect on the gap there is between reality and expectation. If Brutus says he will do something that will make him well, readers would not expect him to end up dead.