Why is Lady Macbeth seen as the fourth witch?
I would not say she is a witch at all. Hecate is the leader of the witches; if anyone is a fourth witch, it is him.
Yes, both the witches and Lady Macbeth plant seeds in Macbeth's murder of Duncan: the witches give Macbeth the idea ("King"), while Lady Macbeth gives him the plan ("murder"), but any other connection after this is tangential or nebulous. The witches and Lady Macbeth inhabit two completely different worlds.
Lady Macbeth is clearly a natural or unnatural (but not a supernatural) creature who tries to employ masculine (not supernatural) means to corrupt Macbeth. She succeeds in convincing him; then, she and her husband suffer physical and psychological problems as a result. Her suicide reveals her mortality.
Lady Macbeth is clearly feminine: one might even say a victim of sexism in Elizabethan society. The witches, on the other hand, have beards. So says Enotes:
Analysts see in the character of Lady Macbeth the conflict between feminine and masculine, as they are impressed in cultural norms. Lady Macbeth suppresses her instincts toward compassion, motherhood, and fragility — associated with femininity — in favor of ambition, ruthlessness, and a lust for power. This conflict colors the entire drama, and sheds light on gender-based preconceptions from Shakespearean England to the present.
The witches have no such lust for power: they are disenfranchised and resort to petty revenge and tricks to satisfy themselves. They make no efforts to rise up the ladder of the Great Chain of Being, unlike Lady Macbeth.
In terms of status, Lady Macbeth is higher on the Great Chain of Being that the witches, who are more or less beggars.
The reason that some people see her this way is because they think she plays a role similar to the three "real" witches. By this, I mean that she makes Macbeth more interested in taking power just like the three witches do. Like them, she helps bring out Macbeth's bad desires and ambitions.
The witches do this by their prophecies. Lady Macbeth does this by encouraging her husband to act on the prophecies -- she pushes him when he seems reluctant. You can see this, for example, when she first hears of the prophecies. She immediately wants Macbeth to be more ruthless in pursuing power.