In act 4, scene 1, the apparitions that appear to Macbeth predict that Macbeth should "Beware Macduff," that "none of woman born shall harm Macbeth," and also that "Macbeth shall never vanquished be until / Great Birnham Wood to high Dunsinane Hill / Should come against him." These three predictions are presented as fate. Later in the same scene Macbeth is shown a line of eight kings, followed by the ghost of Banquo who smiles at Macbeth while pointing to the line of kings. Macbeth understands this as a glimpse into a future that is fated to happen. The future is ruled by Banquo's descendants on the throne that he, Macbeth, currently occupies.

At the end of act 4, scene 1, Macbeth insists that he will, from this moment on, act more decisively, and more impulsively. He says that "The very firstlings of my heart shall be / The firstlings of my hand." This declaration seems to be Macbeth's response to the fates he has just been presented with. It is perhaps a vain attempt to assert his own free will in the face of a fate that has already been set out. This sets up one of the tensions that defines the remainder of the play, between Macbeth's will to direct his own fate on the one hand, and fate's indifference to Macbeth's free will on the other.

In act 4, scene 3, Macduff tries to convince Malcolm to join him in fighting against Macbeth. Macduff bemoans the fact that good people don't take it upon themselves to fight against tyranny. He says that Scotland will continue to bleed because "goodness dare not check thee." In other words, as long as people accept the tyranny that befalls them, without drawing upon their own free will to protest or fight against it, then such tyranny will live on. The idea here is that what can seem like fate, or inevitable and unchangeable, can be overturned if only people believe in their own free will to act. What is perceived as fate only becomes so, or as good as fate, when people don't act.

In the first scene of Act Four, the witches conjure up three spirits that make three separate predictions about Macbeth's future. The first, a disembodied head, warns him to "beware the Thane of Fife," i.e., Macduff. The second, a bloody child, tells Macbeth to

Be bloody, bold, and resolute; laugh to 
scorn The power of man, for none of woman born 
Shall harm Macbeth. 

The third apparition, a "child crowned, with a tree in his hand" assures the king that

Macbeth shall never vanquish'd be until 
Great Birnam Wood to high Dunsinane Hill 
Shall come against him. 

These prophecies fill Macbeth with confidence, suggesting that he is fated to be safe against all opponents. But then he sees a final vision, one of the murdered Banquo and seven of his descendents, all of whom wear the crown of Scotland. This suggests that Banquo's line will be kings, not Macbeth's which deeply disturbs him. Still, takes solace in the prophecies. His confidence is short-lived, however, as he hears that the army approaching his castle disguises itself with limbs from the trees from Birnam, giving the appearance of a forest marching on the Dunsinane. Then, just before his climactic battle with Macduff, he discovers that the Thane of Fife was not, strictly speaking, born of woman, but taken by Caesarian section. His fate, forecast in Act Four, comes true, but not in the way he expected.