Where are Malcolm and Macduff in Macbeth?

In Act IV, Scene 3, Macduff and Malcolm are in England at the court of King Edward the Confessor. Malcolm has been there since fleeing from Macbeth's castle at Dunsinane. Macduff is only newly arrived, having come to Scotland to help raise an army to overthrow Macbeth. Both Macduff and Malcolm are very important characters, but Shakespeare must have felt that he needed to impress them more strongly on his audience, since Macduff had only appeared in the scene in which he discovered Duncan's murdered body and Malcolm, who is destined to become king, had hardly any exposure to the audience at all. 

The part of Act IV, Scene 3 that involves the long conversation between Macduff and Malcolm really accomplishes very little except to make the audience aware that they are important characters who will become antagonists against Macbeth. Malcolm tells Macduff that he doesn't feel qualified to be king of Scotland because of all his vices. He says:

Better Macbeth
Than such an one to reign [as himself].

This seems intended to explain why Malcolm didn't stay at Dunsinane and make an effort to be elected to inherit his father's throne. After all, he was named Prince of Cumberland and was the heir apparent. His election as King of Scotland would have been unanimous. The audience has a right to be suspicious about why Malcolm fled. Evidently he was not only afraid there was a plot against him and his brother, but he didn't really want to be king because he felt unworthy of the office. Macduff has to talk him into asserting his right to rule--and then Malcolm tells him he was lying about his vices in order to test Macduff, whom he suspected of being one of Macbeth's secret agents. 

Devilish Macbeth
By many of these trains hath sought to win me
Into his power, and modest wisdom plucks me
From over-credulous haste. 
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
My first false speaking
Was this upon myself. What I am truly,
Is thine and my poor country's to command:

What he means by "My first false speaking" is that the first time in his life he ever told a lie was when he told Macduff about his nonexistent vices. All this dialogue accomplishes very little except to impress the identities of these two characters on the viewers' minds. (In modern media parlance this would be called "face time.") Later in Act IV, Scene 3, Macduff will learn from Ross that Macbeth has had his soldiers storm Macduff's castle and murder his wife and children along with everyone else inside. This, of course, is important news and augments Macduff's already strong motivation to kill Macbeth in hand-to-hand combat.

O, I could play the woman with mine eyes,
And braggart with my tongue! But, gentle heavens,
Cut short all intermission; front to front
Bring thou this fiend of Scotland and myself;
Within my sword's length set him; if he 'scape,
Heaven forgive him too!

The finale would be much less effective if the audience did not know these two characters better. Malcolm is to become leader of the English army, and Macduff is to have the satisfaction of killing Macbeth on the battlefield and displaying his severed head. At that point Macduff and all the soldiers shout "Hail, King of Scotland!" The audience is satisfied. Malcolm has become not only king but the legitimate and worthy king.