We may, but we don't have to. The concept of Lady Macbeth as a "fourth witch" is one of the standard interpretations of the character, and has been since the play was first written. Not that we must view her in this way, it's just one of the regular critical views. The three witches call forth desires and ambitions from Macbeth that are essentially "bad"- a desire for power at all costs, primarily. Lady Macbeth causes these desires to take actual form in the "real world" of the play, thus giving physical form to the concepts introduced by the witches. She exhibits a ruthlessness, intelligence and determination which to the audience of the time would have been seen as unnatural.
On the other hand, as noted above her mind becomes unhinged by guilt, so she is not a completely evel character. She certainly is not in any specific manner like the three witches.
This entire question is indicative of the play as a whole- the most ambiguous of Shakespeare's works in both action and morality. Ever since it was written (probably 1606), critics and theatre people have consistently expressed dissatisfaction with the abruptness of the ending, the ambiguity of "villians" like Macbeth and his wife who are not totally evil by any means, and the equal ambiguity of "heroes" like Malcolm and Macduff who are far from shining characters. All of the female characters are exagerated and unnatural: the witches are unsettling and sexless; Lady Macbeth is obviously loving toward her husband but ruthlessly merciless; Macduff's wife is so perfect as to be unbelievable.
One of the reasons for all of this is probably the circumstances of the writing. Queen Elizabeth I had recently died and named James Stuart of Scotland as her sucessor. The characters of Duncan, Macbeth, Banquo, etc. were all based on historic personages, one of whom (Banquo) was a direct ancestor of King James, who had just taken on the sponsorship of the Globe. To many Scots, Macbeth was a hero and Macduff and Malcolm considered traitors who delivered Scotland into the hands of the English monarchy. Shakespeare, as an Englishman and a subject of James, would have obviously wanted to present this story in a manner acceptable to his audience and new patron.