What does Malcolm resolve to do by the end of Act 4 of "Macbeth"?
In act 4, scene 3, Malcolm tests Macduff's loyalty before enlisting his help to travel to Scotland in order to defeat Macbeth. After Malcolm admits that he was lying to Macduff about his negative character traits, he informs Macduff that Siward and ten thousand troops are preparing for battle to help them defeat Macbeth. Malcolm is enthusiastic about recruiting Macduff and tells him,
"Now we’ll together, and the chance of goodness Be like our warranted quarrel!" (4.3.138-39)
Ross then enters the scene and explains the dismal environment in Scotland under Macbeth's tyrannical reign before informing Macduff that Macbeth has murdered his family. While Macduff laments about his situation, Malcolm encourages him to use his hate and direct it towards Macbeth. Malcolm tells Macduff,
"Be this the whetstone of your sword. Let grief Convert to anger. Blunt not the heart, enrage it." (4.3.235-36)
Malcolm then tells Macduff to prepare for war because Edward's troops are ready to fight with them. By the end of act 4, Malcolm resolves to get revenge on Macbeth by defeating him in battle with the help of Macduff and Edward's army.
Malcolm and Macduff, in Act 4, scene 3, have just learned that Macbeth had Macduff's wife and children killed. Macduff is stunned almost to being speechless. Malcolm tells Macduff to let the grief Macduff is feeling now be the cornerstone of their desire for revenge against Macbeth. Malcolm says, "Be comforted: / Let's make us medicines of our great revenge, / To cure this deadly grief."
A few lines later, when Macduff asks that Heaven give rest to his slain family and Malcolm says, "Be this the whetstone of your sword: let grief / Convert to anger; blunt not the heart, enrage it." Malcolm is telling Macduff to not let his grief disable him but to let it enable him because they will go to Scotland now, along with help from the English, to overthrow Macbeth.