Early in the play, Malcolm flees to England seeking safety from the murderer who has killed his father. Even though he is the rightful heir to the throne, Malcolm will not return to Scotland. Macduff goes to Malcolm to try to convince him to return and rid Scotland of Macbeth's tyrany. Macduff tells Malcolm that Macbeth means nothing but harm to the Scottish people to persuade him to return. Malcolm, however, says that he does not possess the qualities of a good king: "Nay, had I power, I should pour the sweet milk of concord into Hell." But after Malcolm hears of the treacherous actions of Macbeth, he reconsiders his position and takes back his self-criticism: "I put myself to thy direction, and unspeak mine own detraction." From this point, Malcolm tells Macduff of the strength of the English army and how they can use it to usurp Macbeth.
This scene is important because it establishes Malcolm's qualifications to become the next king of Scotland. When Malcolm is crowned king at the end of the play, the readers are assured that order is restored--evil has been defeated, and good now reigns.
Without Act 4, scene 3, we would be doubtful as to the future of Scotland. Malcolm proves his goodness as well as his intelligence by making sure that Macduff is trustworthy, that he is not going to fall into a trap if he goes back to Scotland with Macduff. At the end of this conversation in which Malcolm falsely claims that he has no kingly virtues, Malcolm admits that he was just testing Macduff, and that he is not greedy or lecherous but instead is truthful and faithful.
We come to admire Malcolm even more when he comforts Macduff after Macduff finds out that Macbeth has had his wife and children murdered. Malcolm encourages Macduff to let this act
Be the whetstone of your sword. let grief
convert to anger; blunt not the heart enrage it.
Malcolm has the zeal, intelligence, goodness, and strength to defeat Macbeth. He is the rightful successor.