The answer to this is, of course, a matter of opinion. Certainly the first scene in Act IV, when Macbeth visits the witches and receives a prophecy that none of woman born can kill him has major consequences for the rest of the play. But I would argue that Act IV Scene 3 is the most important in the act. Two significant events occur in this scene. One is that Macduff learns of the death of his wife and children at the hands of Macbeth's assassins. He is from this point sworn to avenge their deaths, which of course he does at the end of the play. The other event is that Macduff forges an alliance with Malcolm, who ascertains Macduff's purity of motives through pretending to be debauched and overly ambitious. The two are, in fact, planning to invade Scotland when Ross arrives with the news of Macduff's unspeakable loss. With the assistance of Macduff and several other thanes, Malcolm launches an invasion that ultimately leads to the overthrow and the death of Macbeth. So Act IV Scene 3 sets a series of events in motion that lead the play to its conclusion.