Act 1, Scene 4 is important for three reasons. Firstly, King Duncan announces that his eldest son, Malcolm, will become the king of Cumberland, which means that he is now next in line to the throne. This means that Macbeth now faces more opposition if he is to stick with his plot of becoming the King of Scotland.
Secondly, this scene foreshadows the tragic events yet to come in the play. Macbeth begins to become a tragic hero from this scene onwards. This is because we learn that despite King Duncan's desire to reward Macbeth for his bravery and loyalty, which means Macbeth can develop and thrive as a warrior and as a man, Macbeth wants to be the king and will do anything to achieve that. He proclaims that he will have to get rid of Malcolm because he stands in his way of becoming the king:
The Prince of Cumberland! that is a step
On which I must fall down, or else o'erleap,
For in my way it lies.
Thirdly and lastly, this scene documents the gullibility of King Duncan because he trusts Macbeth just like he trusted the Thane of Cawdor, who turned out to be disloyal and treacherous.