How does Shakespeare create a feeling of horror and impending doom at the beginning of Act 4?
Immediately at the beginning of Act 4 in Shakespeare's Macbeth, we feel a sense of dread and horror from the setting and the introduction of the starting characters:
A cavern. In the middle, a boiling cauldron. Thunder. Enter the three WITCHES.
Caverns are typically dark and scary while cauldrons are creepy and associated with wicked means. Adding thunder to the mix creates an eerie atmosphere for the reader or viewer.
The descriptive words the first witch uses (poisoned and venom) as she starts to chant their actions also signify horror. Poison and venom are things people automatically associate with being bad/scary, so we start to feel a sense of dread and doom. The charm itself is cooled at the end with the blood of a baboon, suggesting something vicious and terrible will come from what they're doing since the mixture was composed of various animals' body parts.
Witches themselves are often regarded as an evil sort, using spells and charms to commit bad deeds. So by considering the characters, the setting, and the action—including the fact that we don't know what this concoction will be used for—we feel as if something dreadful is coming in the near future.
The witches always signal doom and gloom in this play. The chanting and mixing animal parts in the cauldron creates the atmosphere of impending doom. Their very presence denotes elements of foreboding and the supernatural. Throughout the play, the supernatural is associated with evil and unnatural transformations. Macbeth and Lady Macbeth become unnatural (contrary to their nature) in order to kill. She asks to be “unsex’d” and he changes psychologically. By the end of the play, both characters are close to madness because of their fear and guilt.
The line that most obviously announces impending doom is when the Second Witch says “Something wicked this way comes.” Macbeth enters right after this. He has become wicked. Act IV, Scene 1 is also when the witches summon ghosts to show Macbeth visions of the future. At the end of this scene, Macbeth announces his plan to kill Macduff’s family.