I think that Shakespeare is showing us action through the eyes and comments of characters other than Macbeth because Macbeth does not know that people hate him and are plotting against him. When we see the thoughts from other people we can get a better understanding of what is going on and what is going to happen.He has because in Act 4 no one likes him and they all want to kill him and he is not as important in Act 4. Macduff and Malcolm are plotting to kill him and their thoughts are more important than Macbeth is.We, the audience are supposed to be feeling sympathy for Macduff because his whole family had just been murdered and for awhile he was starting to feel like the world was going to end.
In act 5 scene 4, Malcom proclaims they shall wear branches when attacking Dunsinane hill. Thus, the forest of Birnam Wood appears to be marching towards Macbeth. This may correspond to the third apparition saying, in act 4, "Macbeth shall never vanquished be until Great Birnam Wood to high Dunsinane Hill Shall come against him" (Shakespeare 4.1.92-95)