This act and scene introduces the witches, Hecate, the goddess of witches (a direct blessing from Hell), shows the grubby nature of evil, and serves both to spur MacBeth on to his murderous scheme to gain the throne of Scotland while simul- taneously prophesying his doom in such a way as to allay his fears about the success of his plans, thus tempting him on.
All the ingredients going into the nasty cauldron represent the lowest nature of villanous behavior. At this point he is still a hero, however brutal. His temptation to murder Duncan to gain the throne comes from many nasty tendencies which have accumulated during his lifetime. He never once questions his own loyalty to king and throne, not to mention Scotland, nor to betrayal of friends. If MacDuff stands in the way, MacDuff and his whole family will be destroyed without a second thought. MacBeth reveals a basic cowardice in his obvious satisfaction with the prophecy which he assumes fortells his avoiding all consequences of his behavior. His satisfaction also reveals his overconfidence and willingness to accept the witches' prophecy without question. Had he any wisdom, he would have considered the source and examined the prophecy more closely.
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.
Act 4, scene i is when Macbeth returns to the witches in order to demand more detailed information about his own future. The purpose of this scene is to: 1) foreshadow; 2) build suspense; and 3) demonstrate that Macbeth's ambition has become uncontrollable.
The predicition of the witches in Act I became true. Therefore, the audience (and Macbeth) believe that these predictions will also come true. The apparitions are foreshadowing what will happen as the rest of the play unfolds:
1) a man not of woman born will appear
2) Birnam Wood will come to Dunsinane
3) Banquo's sons will be king
However, because these messages are so cryptic - particularly the first 2 - the scene itself creates more suspense. Macbeth proclaims to be put at ease by the impossibility of the acts. "Sweet bodements, good!" He says after hearing the first two predictions. But the sight of Banquo's descendants keeps the fear alive, and causes him to decide that
From this moment
The very firstlings of my heart shall be
The firstlings of my hand.
The first action is to seize Macduff's household, and the scene ends with that plan announced.
Macbeth is spiraling out of control in this act. He has become so obsessed with power that he is jumping to conclusions and acting without thinking. Power and ambition, which he tried to ignore early on, have fully corrupted him at this point.