What is the role of the weird sisters in act IV, scene 1?

In this scene, Macbeth returns to visit the Weird Sisters (Witches) to get more information about their original prophecy that he would become king. He wants to find out more particulars not only about what they told him in the beginning of the play, but why they told Banquo that his descendants would be kings. So the role of the witches in this scene is to advance the plot and increase the suspense.

The witches answer his questions. They conjure up horrible ghosts, each of which offers a prediction to dispel Macbeth’s fears. First, a floating head warns him to beware Macduff. Macbeth says that he has already figured that out. Then a bloody child appears and tells him "none born of woman" will harm him. Next, a child with a crown holding a tree tells him that he is safe until Birnam Wood comes to Dunsinane. Finally, a procession of eight crowned kings walks by, the last one carrying a mirror. Banquo’s ghost walks by at the end of the line. Macbeth wants to know the meaning of the final vision, but the witches perform a bizarre dance and  vanish.

The witches' prophecies ease his mind, but as we find out later in the play, what Macbeth has thought to be impossible prophecies- i.e., he can only be hurt by someone that is "not born of a woman" and he will be safe until "Birnum Wood" comes to the castle, turn out to be true because he is killed by one who was born by a Caesarian section procedure and the advancing army is carrying twigs and branches and it looks, indeed, like Birnum Wood coming to get him.

Macbeth shall never be defeated, until
Great Birnam wood shall come to
High Dunsinane hill against him

and

Be bloody, bold, and resolute; laugh to scorn
The power of man, for no one given birth to by a woman Shall harm Macbeth

At the end of the scene, Lennox arrives and tells Macbeth that Macduff has fled to England. Macbeth sends murderers to capture Macduff’s castle and to kill Macduff’s wife and children.