Act IV opens with the witches in a cavern. They have in front of them a cauldron and, together, they are casting a spell by creating a concoction of some rather strange ingredients, including entrails, a newt's eye, a frog's toe, and a lizard's leg to name a few. Macbeth soon enters the scene, and he demands to know how much the witches' prophecies hold truth.

In response to his questions, the witches summon several apparitions out of their spell, all of whom deliver a message to Macbeth. The first apparition, a floating armored head, bids Macbeth to beware of Macduff. The second is the image of a bloody child, who tells Macbeth that "none of woman born / shall harm Macbeth." This seems to imply that, since all people are born from a woman, Macbeth cannot be defeated by anyone. The final apparition is a child wearing a crown and holding a tree. It tells Macbeth that he cannot be defeated until Birnam Wood moves toward Dunsinane Hill. Again, Macbeth's confidence is bolstered by this, as he thinks he cannot be defeated. After all, how can a forest possibly get up and move?

After the three apparitions have delivered their messages, a final image appears to Macbeth. He sees eight kings in a line, the last holding a mirror and, at the end of the line, Banquo's ghost walks. Macbeth is startled to see this, recalling the final prophecy which dictated Banquo's heirs would be kings. Macbeth demands to know from the witches what this final vision means; however, the witches vanish into thin air, just as they have before.

Shortly, Lennox enters to tell Macbeth news about Macduff and how he has fled to England. In response to this, Macbeth decides that he will invade Macduff's castle, killing his wife and children.

The next acts shifts to Macduff's castle in Fife, showing his wife and one of his sons. The young son wonders where his father is, to which his mother replies that he is dead. However, the boy says that, if this were the case, surely his mother would weep for him. The mother then tells her son that his father is actually is a traitor, showing how she feels betrayed and abandoned. A messenger suddenly comes onto the scene, warning Lady Macduff to flee. However, Lady Macduff refuses, feeling she has done no wrong and should not be chased out of her home. A group of murderers then bursts onto the scene, killing the young son. Lady Macduff flees, and the murderers pursue her. The audience is left to assume that they kill her as well.

The last scene of the act moves to Macduff, who has now fled to England to meet with Malcolm. Malcolm does not trust Macduff, feeling as though Macbeth may have sent him as a spy. To test his trustworthiness, Malcolm begins to tell Macduff a series of lies about his own vices. Claiming to be lustful, violent, and greedy, he explains to Macduff that he would make a terrible king. Eventually, Macduff is so overwhelmed with Malcolm's list of flaws that he agrees Malcolm would make a terrible king. He thereby proves himself to Malcolm by showing his loyalty to Scotland and his desire to do right for the country. Malcolm then explains that everything he has just said is a lie and was actually a test. He embraces Macduff as an ally.

Ross then enters the scene, having just arrived in England from Scotland. He tells Malcolm about the problems that Scotland has been having since Macbeth took the throne, and he urges Malcolm to return. Malcolm assures him he will be returning soon with troops lent to him from the English king, Edward. Ross breaks down at this point and tells Macduff that his wife and children have been slaughtered by Macbeth. Macduff is shocked, angered, and filled with grief at this news. As a result, he vows that he will take revenge on Macbeth.

This act marks another development in Macbeth's character. The second round of prophecies has a profound affect on Macbeth. On the one hand, they make him feel invincible, seeming to show that he cannot be defeated. They also cast Macduff as a key threat, which the third scenes reveals to be true. That, coupled with the fact that the final vision shows Banquo's sons still becoming kings, enrages Macbeth. However, the reader should be reminded of the witches' statement at the beginning of the novel that "foul is fair and fair is foul." Nothing in this play is exactly as it seems. Macbeth, however, seems unaware of the fact that the witches could be manipulating him.

In addition, there is also the question of whether or not the witches are showing Macbeth his fate or they are simply presenting him visions that he can choose to act upon. Arguably, he would never have killed Duncan if the witches hadn't planted the thought in his head in act I. In the play, the witches are several times referred to the weird sisters, which can have a double meaning. On the one hand, the witches are strange in their appearance and words. However, the word "weird" may also be derived from the Anglo-Saxon word "wyrd" which meant fate. Their rather ambiguous nature is a subject of some debate: are they truly showing Macbeth his inevitable fate or are they manipulating him?

Regardless, after seeing the four visions, Macbeth is willing to do whatever is necessary to stay in power. When the witches deliver him the second set of prophecies, he decides to take out whoever may stand in his way. However, his decision to murder Mcaduff's innocent wife and children might be seen as the peak of his crimes because they have never done anything to harm Macbeth. All of this shows just how ruthless he has become, a far transition from the man who hesitated to kill Duncan in the first act.

In addition, Macbeth's position as a ruthless leader is a sharp contrast to the conversation that Macduff and Malcolm have in scene three. Their conversation establishes the desirable traits of a king including fairness, temperance, and justice. All of this is a strong reminder that this is exactly what Macbeth is not and highlights his corruption.