Macbeth begins with the witches as they set the scene for the "fair and foul" occurrences and foreshadow the "filthy air" that will pervade Macbeth's thoughts.
By the time we reach Act IV.i. Macbeth has murdered his king and his friend. Hectate the head witch has indicated her disappointment at being omitted from showing "the glory of our art." This is an important contribution to the plot as Hectate indicates that Macbeth's actions are his own and his "vaulting ambition" is what drives him. She has pointed out to the three witches that Macbeth is "spiteful and wrathful" and "loves for his own ends."(III.v.12-13).
Having been prepared for more drama by Hectate, the witches reappear in Act IV and relish in the concoction they are preparing. Undoubtedly there is "double, double toil and trouble"(IV.i.10) as "something wicked comes this way" and Macbeth appears again indicating that Macbeth is the master of his own fate.
Macbeth's desperation is apparent and his descent into madness confirmed as he seeks confirmation that having committed these crimes, "the worst means the worst" (III.iv.134), he can be assured. Macbeth is still afraid but the witches prophesies mean he can "tell pale-hearted fear it lies, and sleep in spite of thunder."(Iv.i.85-86) . Macbeth's biggest fear is at being caught as his deeds are "scanned" before he's had a chance to act.
The plot develops in Act IV.i. as the witches inspire Macbeth as they"hath harp'd my fear aright." Macbeth's need for more then leads the witches to basically seal his fate when they tell him that "none of woman born shall harm Macbeth"(IV.i.80). Macbeth is encouraged as more ridiculous claims - Birnam Wood - excite him and he finally demands information about Banquo's sons. Not content with the remark from the witches, he intends to take matters in to his own hands "before this purpose cool." (154)
Macbeth's character has seen a major change during this scene as he becomes more resolute in his actions and determination. He is able to set his fear aside and there is no indication of doubt or conscience. We are prepared for the shedding of more blood before this conflict is finally resolved when Birnam Wood does indeed advance on Macbeth!