Act 4, Scene 1
The witches circle a cauldron, mixing in a variety of grotesque ingredients while chanting "double, double toil and trouble; / Fire burn, and cauldron bubble" (10-11). Hecate appears, they sing all together, and Hecate leaves. Macbeth then enters, demanding answers to his pressing questions about the future. The witches complete their magic spell and summon forth a series of apparitions. The first is an armed head that warns Macbeth to beware the Thane of Fife (Macduff). The second apparition is a bloody child, who tells him that "none of woman born / Shall harm Macbeth" (96-97). This news bolsters Macbeth spirits. The third apparition is a crowned child with a tree in its hand, who says that "Macbeth shall never vanquished be until / Great Birnam Wood to high Dunsinane Hill / Shall come against him" (107-09). This cheers Macbeth even more, since he knows that nothing can move a forest. Macbeth proceeds to ask his last question: will Banquo's children ever rule Scotland?
The cauldron sinks and a strange sound is heard. The witches now show Macbeth a procession of kings, the eighth of whom holds a mirror in his hand, followed by Banquo. As Banquo points at this line of kings, Macbeth realizes that they are indeed his family line. After the witches dance and disappear, Lennox enters with the news that Macduff has fled to England. Macbeth resolves that he will henceforth act immediately on his ambitions: the first step will be to seize Fife and kill Macduff's wife and children.
Act 4, Scene 2
At Fife, Ross visits Lady Macduff, who is frightened for her own safety now that her husband has fled. He reassures her by telling her that her husband did only what was right and necessary. After he leaves, Lady Macduff engages her son in a conversation about his missing father. The little boy demonstrates wisdom well beyond his years. A messenger interrupts them with a warning to flee the house immediately. But before Lady Macduff can escape, murderers attack the house and kill everyone including Lady Macduff and her son.
Act 4, Scene 3
Macduff arrives at the English court and meets with Malcolm. Malcolm, remembering his father's misplaced trust in Macbeth, decides to test Macduff: he confesses that he is a greedy, lustful, and sinful man who makes Macbeth look like an angel in comparison. Macduff despairs and says that he will leave Scotland forever if this is the case, since there seems to be no man fit to rule it. Upon hearing this, Malcolm is convinced of Macduff's goodness and reveals that he was merely testing him; he has none of these faults to which he has just confessed. In fact, he claims, the first lie he has ever told was this false confession to Macduff. He then announces that Siward has assembled an army of ten thousand men and is prepared to march on Scotland.
A messenger appears and tells the men that the king of England is approaching, attended by a crowd of sick and despairing people who wish the king to cure them. The king, according to Malcolm, has a gift for healing people simply by laying his hands on them.
Ross arrives from Scotland and reports that the country is in a shambles. When Macduff asks how his wife and children are faring, Ross first responds that they are “well at peace” (180). When pressed further, he relates the story of their death. Macduff is stunned speechless and Malcolm urges him to cure his grief by exacting revenge on Macbeth. Macduff is overcome with guilt and sorrow from the murders that occurred while he was absent. Again Malcolm urges him to put his grief to good use and seek revenge. All three men leave to prepare for battle.
As the act opens, the witches carry on the theme of doubling and equivocation that threads throughout the play. As they throw ingredients into their cauldron, they chant "double, double, toil and trouble"—a reminder that their speech is full of double meanings, paradox, and equivocation (IV i 10). The apparitions that the witches summon give equivocal messages to Macbeth, and they appear to know quite consciously that he will only understand one half of their words. Although Macbeth himself has previously acknowledged that "stones have been known to move and trees to speak" (III iv 122), the apparitions give Macbeth a false sense of security. He takes the apparitions' words at face value, forgetting to examine how their predictions could potentially come true.
The theme of doubling is amplified when the witches summon the "show of kings." Each king who appears looks "too like the spirit of Banquo," frightens Macbeth with their resemblance (IV i 128). For Macbeth, it is as if the ghosts of Banquo have returned to haunt him several times over. In the procession of kings, Macbeth also notes that some carry "twofold balls and treble scepters"—as if even the signs of their power have been doubled.
On a historical note, it is generally thought the eighth king holds up a mirror in order to pander to James I. This last king—the eighth-generation descendant of Banquo—is none other than a figure of James I himself. He thus carries a mirror to signal as much to the real James I, who sits at the forefront of the audience. A similar moment of pandering occurs when Malcolm notes that the king of England has a special power to heal people affected by “the evil” (147). In various subtle ways, Shakespeare complimented King James I—a legendary descendant of Banquo and author of a book on witchcraft (Daemonologie ).
James I is not the only character who is doubled in Macbeth. Throughout the play, characters balance and complement each other in a carefully constructed harmony. As a man who also receives a prophecy but refuses to act actively upon it, Banquo serves as sort of inverse mirror image of Macbeth. Although he has troubled dreams like Macbeth, his arise from the suppression of ambitions whereas Macbeth's arise from the fulfillment thereof. Other major characters, including Malcolm, Macduff, and Lady Macbeth, can also be seen as foils or doubles for Macbeth. Particularly interesting is the case of Lady Macbeth, who in some sense “switches roles” with Macbeth as the play progresses. Whereas she first advises Macbeth to forget all remorse and guilt, Lady Macbeth becomes increasingly troubled by her own guilt as Macbeth begins to heed her advice.
Another form of doubling or equivocation is found in the theme of costumes, masks, and disguises. While planning Duncan's murder, Lady Macbeth counsels Macbeth to "look like the innocent flower, / But be the serpent under't"—to "beguile the time" by disguising his motives behind a mask of loyalty (I v 61). After the murder, Lady Macbeth paints the bodyguards' faces with a mask of blood to implicate them. Similarly, while preparing to kill Banquo, Macbeth comments that men must "make [their] faces visors to [their] hearts, / Disguising what they are" (III ii 35-36). Thus when Malcolm tests Macduff's loyalty, he begins appropriately by saying that "all things foul would wear the brows of grace" (IV iii 23). Even the most foul of men—perhaps like Macbeth and the murderers—are able to disguise themselves. Just as the witches’ equivocation covers up the true harm within their alluring words, disguises and masks hide the inner world from the outer.
Finally, during the scene in which the murders occur, Lady Macduff reflects the bird symbolism that began in Act 1. When Lady Macduff complains to Ross about the abrupt departure of Macduff, she states: "the poor wren / The most diminutive of birds, will fight, / Her young ones in her nest, against the owl" (IV ii 9-11). Her metaphor comes to life when she and her son are attacked by Macbeth's men. Macbeth, as earlier established, is identified with the owl; so Lady Macduff, trying to protect her son, becomes the wren in a realization of her own figure of speech. It is with particular pathos that the audience sees Macduff’s precocious son fall prey to the swords of Macbeth’s ruthless murderers.