In a cavern, the weird sisters throw awful ingredients such as "eye of newt and toe of frog" (4.1.14) into a cauldron full of a boiling brew. Hecate arrives, and all dance and sing. One witch cries out "Something wicked this way comes" (4.1.62): Macbeth enters. He commands the witches to answer his questions.
The witches conjure up three apparitions. First, a floating head appears and tells Macbeth to beware Macduff.
The head symbolizes either Macduff's rebellion or Macbeth's fate.
Next, a bloody child appears. The child says that "no man of woman born / Shall harm Macbeth" (4.1.95-96).
The bloody child symbolizes Macduff's birth by caesarian section.
Finally, a child wearing a crown and holding a tree appears. It says that Macbeth will not be defeated until Great Birnam Wood marches to Dunsinane Hill. Macbeth is pleased: since forests don't march, he must be invincible!
The child with crown and tree symbolizes Malcolm.
Macbeth wants to know one more thing: will Banquo's heirs have the throne? The witches perform a final conjuring. Eight kings appear walking in a line, the eighth holding a mirror, and all of them followed by Banquo's ghost. Macbeth, furious at this sign that Banquo's heirs will get the throne, demands answers. But Hecate mocks him and the witches vanish.
The king holding the mirror symbolizes King James who ruled England when Shakespeare wrote Macbeth, and whose family traced its ancestry back to Banquo.
Lennox enters. He brings word that Macduff has fled to England. In an aside, Macbeth scolds himself for failing to kill Macduff when he wanted to earlier. He vows in the future to act on every impulse, and decides to attack Macduff's castle and kill anyone connected to him: servants, wife, and children.
Ambition and fear have pushed Macbeth that final step: he is no longer targeting just his political enemies, but also their innocent families. Macbeth is now truly a monster.