Although Macbeth’s downfall becomes somewhat evident in Act III (when he is being taunted by Banquo’s ghost), I think Act IV represents the breaking point for Macbeth, as well as his rule over Scotland. Scene I opens up with the witches concocting an elaborate recipe and later telling Macbeth of his future – they give him news that (at the time) seems to be positive, as usual, for the King. However in Scene II, it is clear that Macbeth, as well as the entire world around him has been brought to total devastation. Out of fear, Macduff has left his wife and children without notice just to get away from the “abhorred tyrant.” It seems that Macduff and Malcolm have some hostility between them (I actually thought they were going to fight in Act IV, Scene III), yet they team up in order to overtake the diminishing Macbeth.
Unrelated observation: In Scene II, Macduff’s son seems extremely thoughtful and well-versed for a child. He participates in a very peculiar exchange with his mother.
The fourth act is generally a turning point in Shakespearean tragedy, and your comments about Lady Macduff’s conversation with her son are well taken. How might that reflect the complicated view of time in this play?