What purpose does Act 4 scene 1 serve in the play "Macbeth"? And what does it say about Macbeth's character and the themes of the play?

In Act IV, Scene I, Macbeth seeks out the  witches, unlike the first encounter, which was initiated by the witches.  This time he is determined to get answers from them regarding his reign as king. 

Macbeth is obsessed with keeping his crown.  His personality has been dramatically altered since the first prophecy was given to him, so now, crowned king, as the witches predicted, Macbeth struggles with questions and threats that he perceives.

The main theme of the play, unchecked ambition and its consequences is fully expressed in this scene, as Macbeth demands information about the future. 

He is given information that makes him feel very over confident about his reign as king.  This is of course the intention of the witches, to fill Macbeth with such arrogance and confidence that he will not see his doom coming.

After this scene, Macbeth is even more determined to protect his crown, even though he feels disgusted when the witches show him the parade of eight kings followed by the ghost of Banquo, which indicates that his heirs will command the throne for generations. It only serves to heighten Macbeth's determination to murder anyone who appears to be a threat to his crown.

After he receives the second prophecy, Macbeth's murderous behavior becomes irrational, he sends killers to slaughter MacDuff's family, to strike at the absent Macduff.