What is the theme for Act 4 in The Tragedy of Macbeth?
One theme of Act IV is deception, the idea that appearances can be deceiving. In scene 1, the Weird Sisters seem to offer Macbeth some assurance that he will be safe indefinitely because they tell him that
"none of woman born / Shall harm [him]" and, further, that "Macbeth shall never vanquished be until / Great Birnam Wood to high Dunsinane Hill / Shall come against him."
However, rather than assurances of safety, these are really just confusingly and mysteriously-worded statements of fact. The Sisters intend to deceive Macbeth in order to make him feel "secure," as Hecate previously said, and thus he will become overconfident and careless.
In scene 3, Malcolm purposely portrays himself as the most excessive, morally perverted contender for the throne in order to test Macduff's honesty. He fears that Macbeth has sent Macduff to lure him home so that Macbeth can kill him. As long as Macduff continues to protest and assure Malcolm that he is still a better choice than Macbeth, Malcolm doubts his sincerity; however, when Macduff finally declares that Malcolm is not only unfit to rule, but unfit to live, Malcolm knows that Macduff is honest, and he can drop the ruse. However, Malcolm deceives Macduff in order to test him, and thus this scene also supports the theme of deception.