In Shakespeare's Macbeth, the new and particularly significant characterization introduced in Act Four, scene one, is the true depth of Macbeth's depravity: both in the eyes of mankind and that of God.
The audience has already learned that while the other witches toyed with Macbeth for their own enjoyment, Hecate is much, much darker in her intent. Making a human's life chaotic and miserable is not her end game: Hecate wants Macbeth's soul.
This scene would not have raised eyebrows in Shakespeare's time: Elizabethans very much believed in witchcraft and other forms of the supernatural (as seen with the floating dagger before Duncan's murder or in Banquo's appearance to Macbeth after he is murdered). Elizabethans had a healthy fear of evil and Shakespeare was wise enough to include this element for it was vastly intriguing to the people of his day. And, so too, they would not have been surprised either by Hecate's desire to annihilate Macbeth or by his fate, brought on by trafficking with the evil denizens of the heath—for they believed that Macbeth had sold his soul to be King of Scotland.
As the witches announce Macbeth's arrival, note that they (these servants of the devil) recognize Macbeth as wicked. This further characterizes Macbeth in providing affirmation that Macbeth's transition from the loyal and valiant man of Act One to a despicable creature is complete—he is steeped in blood and there is (as he has noted already) no turning back—further he shows no regret.
By the pricking of my thumbs,
Something wicked this way comes… (44-45)
The audience also sees the extent to which Macbeth has forgotten what he once knew in his heart to be true. At the beginning of the play, when the witches' prediction that he will become Thane of Cawdor is realized, Macbeth is confused but also frightened:
This supernatural soliciting
Cannot be ill, cannot be good…
If good, why do I yield to that suggestion
Whose horrid image doth unfix my hair
And make my seated heart knock at my ribs,
Against the use of nature? (I.iii.141-142, 145-148)
While Macbeth welcomes the new title, and what would seem to promise his rise to the throne, Macbeth (who is a proven hero on the battlefield) is so frightened by his brush with the witches and their seeming power, that his hair stands on end.
By Act Four, Macbeth has no fear of the witches. This is a disastrous mistake, for he believes that he is a match for them, that he can control them. Hecate has already noted that a man's "false sense of security" will bring about a man's downfall:
And you all know security
Is mortals’ chiefest enemy. (III.v.32-33)
It is obvious that Macbeth does, indeed, have a sense of security that is not grounded in fact: the witches have led him to believe they serve him, but nothing could be less accurate. He is too deluded to notice—when he tries to ask a question of the first apparition, the witch tells Macbeth that it cannot be commanded.
We see more clearly that Macbeth will never relinquish his crown. The second apparition tells Macbeth that "none of woman born" (89) can harm him, but he still vows to kill Macduff just to be safe. We might infer that this shows some skepticism toward the witches, but Macbeth still relies on their words until the woods move.
Macbeth's wisdom has fled: when the witches show him a vision of Banquo's descendants, he curses them, but still does not turn away from what he wants to believe: that he is invincible.