Malcolm does become king at the very end of the play, when Macduff presents him with Macbeth's severed head, and he seems to possess the "king-becoming graces" which he tells Macduff, at first, that he does not have. It is all part of a test; Malcolm is concerned that Macduff is really in league with Macbeth and has come to England to bring Malcolm home so that Macbeth can kill him. He describes the graces a king must have as:

justice, verity, temp'rance, stableness,
Bounty, perseverance, mercy, lowliness,
Devotion, patience, courage, [and] fortitude (4.3.108-110).

We learn soon that Malcolm dearly loves his country and that he is actually an upright and moral person. When Macduff proclaims that Malcolm should not come home to rule, based on the falsehoods Malcolm tells about himself, he has passed Malcolm's test. Malcolm, now, truthfully tells Macduff,

I am yet
Unknown to woman, never was forsworn,
Scarcely have coveted what was mine own,
At no time broke my faith, would not betray
The devil to his fellow, and delight
No less in truth than life. My first false speaking
Was this upon myself. What I am truly
Is thine and my poor country's to command (4.3.144-151).

In other words, then, he is truthful, moderate, quite stable, devoted to Scotland, patient, and courageous. He is prepared to return, bravely, to Scotland to fight on behalf of his country, to save it from Macbeth's tyranny. He is not selfish and power-hungry like Macbeth is. In Malcolm's final speech, which ends the play, he again shows that he is just and fair and that he is not covetous of power.

In the passage, Malcolm is not referring to a particular king; he is naming all the characteristics that a good king must possess. Within the context of the play, they would most accurately describe King Duncan, whom Macbeth had murdered. Even as he prepares to kill Duncan, Macbeth recognizes what an excellent king Duncan is. Again, in reference to the list, Banquo, Malcolm, Macduff, and Donalbain are not kings. Malcolm is, however, the rightful heir to the throne following his father Duncan's death.