What a wonderful idea.
Lady Macbeth, from William Shakespeare's tragic play Macbeth, could be regarded as the fourth witch (if Hecate is not being counted).
The witches are the first characters shown in the play. This is done to set the ominous and dark mood of the play (given it is a tragedy). Every time readers, or watchers, of the play come in contact with the witches, nature is mimicking the mysteriousness of the witches. They are indeed wicked (also denoted by the lines speaking about the sailor).
Lady Macbeth is a very ominous and devious character (like the witches). She, upon hearing about the prophecy, is determined to gain the crown for Macbeth. She cries out to the spirits to "unsex me here." This certainly seems to be something a witch would do and not a notable lady of the courts.
Outside of her evoking of the spirits, Lady Macbeth is not concerned (in the beginning) about the murder of Duncan. She knows that Duncan must die for Macbeth to become king and she will not stop until he is.
Her behavior, her mind set, and her outright menacing behavior could all justify Lady Macbeth as the fourth witch of the play.