In Act IV, the witches summon apparitions, their "masters," to give Macbeth the predictions. The first apparition tells Macbeth to "beware Macduff" and then disappears. Macbeth essentially says 'thanks for the warning.' But when Macbeth asks to have the first apparition tell him more, the first witch says that the apparition "will not be commanded." Thus, this is all Macbeth will get regarding the warning about Macduff.
The second apparition appears and tells Macbeth that "none of woman born / Shall harm Macbeth." (IV.i.96-97) Macbeth then asks why he would need to fear Macduff at all, thinking that Macduff was born from a woman. However, Macbeth wants to make sure, so he decides to have Macduff killed:
Then live, Macduff--what need I fear of thee?
But yet I'll make assurance double sure,
And take a bond of fate thou shalt not live, (IV.i.98-100)
The third apparition tells Macbeth to be brave and not to worry because he (Macbeth) will never be killed until Great Birnam Wood comes to Dunsinane Hill. Macbeth notes that this is impossible, noting that no one can move a forest. This is, therefore, a good omen, a "sweet bodement."
Finally, Macbeth threatens the witches with a curse unless they tell him if Banquo's descendants will ever rule. Eight apparitions appear as a line of kings, the last holding a mirror ("glass") reflecting Banquo.