I need help writing a new scene based on Lady Macbeth that has a soliloquy and falls between Act 4, Scene 3 and Act 5, Scene 1.
Essentially, it sounds to me that you are seeking to build a bridge between the brutal murders in Act 4, sc. iii and the emerging pattern of Lady Macbeth sleepwaking and "strange behavior" in the first scene of the final act. I think that one potential idea that could emerge would be to give more detail to the debilitation of Lady Macbeth. For example, a soliloquy could emerge where Lady Macbeth hears the screams of Lady Macduff and her child. We have not really seen Lady Macbeth focus in on the murders and foul deeds from a personalized capacity. A potential idea for a soliloquy could be to focus in on this point. It might be persuasive because Lady Macbeth could respond as a wife and potential mother. Given her "unsexed" speech earlier in the drama, the soliloquy could be harnessing her own sex as its focal point.
The exposition of this new Act 4, scene iv could take place while Lady Macbeth is asleep. As she sleeps, she dreams of the murders of Lady Macduff and her son. She should be troubled as the nightmare of seeing them murdered at her husband's command and hand grabs at her. She should hear the screams, the hacking of the flesh, and the nightmare should see Lady Macbeth seeing Lady Macduff screaming for her dead child and her own death. Lady Macbeth's nightmare should be powerful enough to shake her to her core, helping to enhance her destabilization.
In her soliloquy, there should be a confessional tone to it, something to reflect her horror at the beast she has unleashed in her husband. A potential starting point for this soliloquy could focus in on the child's and mother's screams of desperation. It could open with the screams of the child and mother and build from there. It has to be something that enables her own culpability in speaking to the audience. Her soliloquy has to convey the sense of terror and sense of guilt she feels, something that is conveyed in the "strange behavior" in the first scene of the final act. I believe that the soliloquy should be a bridge that helps to convey in her own words the behavior that plagues her in Act V. In being able to develop a soliloquy in this vein, a very interesting aspect of her characterization can be revealed.