What event prompts soliloquy in scene 4 of act 4?

In scene 4 of act 4 of Macbeth, Macbeth delivers a soliloquy after the news of Lady Macbeth's death. This event prompts Macbeth's soliloquy, which is often referred to as the "Tomorrow" speech. In this soliloquy, Macbeth reflects on the meaninglessness and insignificance of life in the face of death.