The event in Act IV that foreshadows Macbeth's eventual down fall is that;
Macduff discovers Macbeth has murdered his family and vows to kill Macbeth himself. This is used to foreshadow an imminent conflict between Macbeth and Macduff.
Explanation;
Foreshadowing is the use of indicative word or phrases and hints that set the stage for a story to unfold and give the reader a hint of something that will happen without revealing the story, or interfering with suspense.
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