Final answer:
The fourth apparition in Macbeth is a vision of a procession of kings holding mirrors. It is especially terrible to Macbeth because it confirms that he will be succeeded by Banquo's descendants.
Step-by-step explanation:
The fourth apparition in Macbeth is a vision of a procession of kings, each holding a mirror. This apparition is especially terrible to Macbeth because it shows him that his downfall is inevitable and that he will be succeeded by Banquo's descendants. It confirms the prophecy that Banquo's sons will be kings, which threatens Macbeth's hold on power.