Macduff and his family dominate Act 4, while Macbeth has all but disappeared from the play at this point. Consider the following:
Macbeth has 117 lines in each Act 1 and Act 2
He has 260 lines in Act 3 alone, yet he speaks a mere 73 lines in all of Act 4.
Why do you think Shakespeare is showing us action through the eyes and comments of characters other than Macbeth?
Why has Macbeth lost his privilege of delivering long soliloquies to the audience in which he ponders his dilemmas?
Who are we, the audience, supposed to be feeling sympathy for at this point?