Considering all the relevant instances and happenstances in the entire play,can Lady Macbeth be called the "fourth witch"?Please justify your choice.
Macbeth is a play full of what you call "happenstances"; however, it's unlikely that Lady Macbeth is actually one of the witches. She first hears her husband's new through a letter, and she displays a range of emotions in her soliloquy--but none of them involve a fore-knowing or prior knowledge of the incidents related. If she were a witch, she would already have known and there would be no surprise. Lady Macbeth is also singularly caught unaware by other events in the play: Macbeth's killing of the guards, her husband's plan to kill Banquo, Macbeth's seeing the ghost of Banquo, and more. The witches appear to have some kind of prophetic power, something Lady Macbeth clearly does not have. In the end, though, it is her obviously guilty conscience which convinces me she is not the fourth witch.
I don't see Lady Macbeth as "the fourth witch." In my mind, the fact that she suffers almost as much as Macbeth, and perhaps even more, indicates that she cannot be seen as a witch. She lacks the power of prophecy and is not in a removed position as the witches are. Lady Macbeth has a vested and personal interest in what Macbeth does. Accordingly, as he descends down the path of personal destruction, Lady Macbeth does not remain as a party that is not separate from this position of personal abyss. She suffers through guilt and remorse, through self hate and the inability to interact with the world around her. Additionally, as Macbeth becomes more dependent on the witches, he actually becomes less emotionally connected with his wife, indicating that she is not seen as a witch.