What event in Act IV foreshadows Macbeth's eventual downfall?

In Act IV of Macbeth, Lady Macduff and her children are murdered which is supposed to display to the reader, how ruthless Macbeth has become through the course of the play. The end of the act is used to foreshadow an imminent conflict between Macbeth and Macduff.

Answer;

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.