In Act IV, Macbeth grows over-confident when he learns of the weird sisters newest prophecies - prophecies that seem so unnatural that he is safe from any possibility that they will come true. For example, when the sisters say that "none of woman born/Shall harm Macbeth" Macbeth's reply is, "Then live, Macduff: what need I fear of thee?" (4.1). Next, the sisters inform Macbeth that he will not be vanquished until Birnam Wood manages to march 12 miles to Dunsinane Hill...another unnatural and therefore impossible (according to Macbeth) occurrence because it violates the natural order of things.
However, since the natural order of things--or the Elizabethan worldview that states that all things are ordered (from God down to the smallest rock)--was upset early in the play with the killing of a king, Macbeth should be very afraid! These prophecies in Act IV further the themes that concern the natural order and the chaos that ensues when it is disturbed.
Both order versus chaos and natural world versus unnatural world are dominant themes throughout "Macbeth," but we definitely see it strongly throughout Act IV.
Act IV begins with the witches and their famous "Double, double, toil and trouble" scene, which is the ultimate in chaos and unnaturalness in this play. Macbeth arrives, demanding that the hags show him what is going to happen. This was a guy who, when the play started, was being hailed as the great war hero, a loyal subject of King Duncan, and now we see that he has fallen so far that he is resorting to gaining information from witches and apparitions.
Scene 2 shows us a different scene - a household scene between Lady Macduff and her son. This is a 180 degree shift from the unnaturalness contained in scene 1, but soon we see that the order and natural world contained in the Macduff household is about to come crashing down...caused by the chaos that Macbeth has jumped into. He sends men to murder this woman, her child, and servants, which again is horrible unnatural.
Scene 3 then shows us two good men - Malcolm and Macduff - who are taking measure of one another to see if they can be trusted. Malcolm is unsure of Macduff until some questioning has taken place, and we see that the rightful heir to the throne (Malcolm) is going to try to go back and set things right in Scotland.
This act is full of chaos and order both, and definitely shows the two as opposing forces to one another.