Act IV, scene three occurs in England, where MacDuff and Malcolm wait outside King Edward's palace. The premise of this particular act is that Malcolm hesitates to place his trust in MacDuff, so he attempts to test MacDuff's love of Scotland by pretending to have numerous terrible vices. Here is where Macbeth enters the conversation, because as horrid as Malcolm pretends to be, MacDuff is sure that he cannot be as bad as Macbeth:
"Not in the legions
Of horrid hell can come a devil more damn'd
In evils to top Macbeth" (IV.iii.63-65).
Shakespeare uses MacDuff and Malcolm's conversation to reveal the building opposition to Macbeth; Macbeth's character certainly feels involved in Act IV, scene three, because he is one of the central topics of discussion between Malcolm and MacDuff, especially after MacDuff learns of his family's deaths at the hands of Macbeth's henchmen. Macbeth's evil presence darkens the end of the scene as MacDuff bemoans his family's undeserved fate and vows for revenge:
"Bring thou this fiend of Scotland and myself;
Within my sword's length set him; if he 'scape,
Heaven forgive him too!" (269-271).