What does Malcolm resolve to do by the end of Act 4 of "Macbeth"?

Malcolm and Macduff 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, "Let's make us med'cines of our great revenge / To cure this deadly grief."  A few lines later, when Macduff asks that Heaven give rest to his slain family, Malcolm says, "Be this the whetstone of your sword.  Let grief / Convert to anger; blunt not the heart, enrage."  Malcolm is telling Macduff to not let Macduff's grief disable him but to let it enable him.  They will go to Scotland now, along with help from the English, to overthrow Macbeth.