I like your interpretation of Lady Macbeth as the fourth witch, or at least a co-conspirator of the whole process of instigating Macbeth to kill the King. Lady Macbeth plays a crucial role in convincing her husband that he must seize the opportunity put before him by both the prophecy and the King's visit to his home.
Macbeth, although consumed with a desire for power, actually decides not to kill the King.
"We will proceed no further in this business: He hath honor'd me of late, and I have bought35) Golden opinions from all sorts of people, Which would be worn now in their newest gloss, Not cast aside so soon."
He tells his wife that the plot is over, let it go. She persists.
"Such I account thy love. Art thou afeard
To be the same in thine own act and valor
As thou art in desire? Wouldst thou have that(45) Which thou esteem'st the ornament of life
And live a coward in thine own esteem,
Letting “I dare not” wait upon “I would”
Like the poor cat i’ the adage?"
She tells him that he is less than a man if he is too much of a coward to kill the King.
So Lady Macbeth does use her powers of persuasion to convince her husband that he must kill the king, if he loves her he would kill the king, seize the throne and make her have her heart's desire, to be queen.
When you add up the prophecy which stirs Macbeth's deepest desire for power and recognition and add in Lady Macbeth's convincing argument, he has been preyed upon by four witches. He is manipulated by two different types of magic, one that is designed to bring out his desire for power and greed and envy, the dark passions. And the other, stirred by his wife who assaults his ego, virility and capacity as a man and a husband to persuade him to commit murder, which she sees as opportunity, not as evil.
Lady Macbeth leads her husband the rest of the way to the dark side. She has her limits too though, after the murdering gets out of control and Lady Macduff and her family are killed by Macbeth's thugs, she finds her conscience.
"Right" is not exactly the term I would use. She may certainly be viewed as an extension of the witches, or as a symbolic fourth witch, because of her mental strength, power over her husband, and ruthlessness. She calls forth from Macbeth the same desires that the witches called forth, ambition, lust for power, etc. In some respects, Lady Macbeth and her husband are a metaphor for the story of Adam and Eve, the woman persuading her mate to commit sin. But Lady Macbeth is not the culprit or perpetrator of the crime, not the originator of sin any more than Eve, nor any more than the witches themselves.
Macbeth's acting on the forbidden desire is, in this play, what constitutes being a man- and Lady Macbeth calls this forth by daring him to stop being a child. But when he does, she loses her power over him. This is a strangely misogynistic play, where the women are all somewhat unnatural creatures except for Macduff's wife, who is the paragon of the good woman. The women are all removed by the end of the play, leaving only a world of heroic warrior men.
Macbeth and his wife exhibit, for all their negative qualities, an intimacy and care for one another (and understanding of each other) far beyond the normal married pair in Shakespeare's works. But Lady Macbeth's strong will, quick mind and initial power over her husband most likely was interpreted by the contemporary audience as somewhat supernatural.
I think it is right to consider Lady Macbeth as the fourth witch ,this is so because her persuasion the play of macbeth is so strong that Macbeth is not able to resist. I don't think it is right for a wife to persuade her husband so much , that he is powerless to answer back to her request,and this is why I truly agree that she can be considered as the fourth witch in the play of macbeth
So, you're suggesting...
When a man is tempted by power, he is a tragic hero.
But when a woman is tempted by power, she is a witch.
Lady Macbeth is not a witch. Powerful women are not "unnatural aberrations of gentle female nature in league with the devil." Throughout history Men have had great difficulty with accepting powerful and/or intelligent women, so they have often labelled them as unnatural monsters.
In history, 'Witches' were often no more than bossy, intelligent women who questioned men's decisions. Of course Lady Macbeth is not a witch. She's a heartless, bloodthirsty, power-hungry woman, not a spooky monster.