How do the characters in scene 4 contribute to plot of

The two witches introduce the idea of fate and prophecy; it's here that we first hear the three famous phrases that eventually lead to Macbeth's downfall — and they don't come solely from the Weird Sisters' incantations, either. Macbeth himself completes the idea when he says, “Stars, hide your fires; let not light see my black and deep desires.” As if summoned, Banquo then appears—which suggests his presence is perhaps no accident.

When Banquo and Macbeth encounter the witches, the prophecies they receive provide the inciting incident of the plot - the trigger that sets all subsequent events in motion, leading towards the overarching conflict ( Macbeth's desire for power and murder of Duncan )and ultimate tragic fate