Who is Duncan referring to in his speech in Act I, Scene 4, before Macbeth, Banquo, Ross, and Angus enter?

Just before Macbeth, Banquo, Ross, and Angus enter in Act I, Scene 4 of Macbeth, King Duncan says:

There's no artTo find the mind's construction in the face.He was a gentleman on whom I builtAn absolute trust.

Duncan is referring to the Thane of Cawdor who has just been executed for high treason. As Scene 4 opens, Duncan asks his son Malcolm:

Is execution done on Cawdor? Are notThose in...

Just before Macbeth, Banquo, Ross, and Angus enter in Act I, Scene 4 of Macbeth, King Duncan says:

There's no art
To find the mind's construction in the face.
He was a gentleman on whom I built
An absolute trust.

Duncan is referring to the Thane of Cawdor who has just been executed for high treason. As Scene 4 opens, Duncan asks his son Malcolm:

Is execution done on Cawdor? Are not
Those in commission yet returned?

Malcolm informs his father that Cawdor is dead and relates what he has heard about how Cawdor died bravely and begged Duncan's pardon for his treachery.

It is ironic that Macbeth enters just at the point where Duncan says that there is no way of reading a person's mind from his facial features or facial expressions. Macbeth is already the new Thane of Cawdor, and he is thinking of assassinating Duncan in order to replace him as king of Scotland. The ensuing conversation between Duncan and Macbeth is understood by the audience as requiring Macbeth to hide his true thoughts, feelings and intentions. When Duncan announces that he is naming his son Malcolm the Prince of Cumberland, Macbeth must continue to hide behind a rigid mask, but he has a strong internal reaction to this news because it makes Malcolm the official heir apparent to the throne. Macbeth tells himself in an aside to keep the truth hidden:

Stars, hide your fires;
Let not light see my black and deep desires.
The eye wink at the hand; yet let that be
Which the eye fears, when it is done, to see.

Only the audience knows what is going on inside Macbeth. He is learning to be treacherous. It is almost as if he has inherited the duplicitous nature of Cawdor along with his title.

Macbeth would have a hard time becoming king if he killed Duncan but left Malcolm alive. Then there was Donalbain, the younger son, who would seem to be the natural successor to the throne if Malcolm were eliminated. Shakespeare did not know how to deal with this problem. He had enough to think about in staging the bloody assassination of Duncan. It really looks as if Shakespeare was only thinking one scene ahead at a time. He didn't know what Macbeth was going to do about Malcolm and Duncan. Maybe Macbeth would murder all three in their beds that night. Shakespeare was counting on his own genius to handle the problem with the boys after Macbeth and his wife had disposed of their father. The final solution was to have Malcolm and Donalbain decide to flee for their lives, Malcolm to England and Donalbain to Ireland. In Act II, Scene 3, Malcolm tells his brother:

This murderous shaft that's shot
Hath not yet lighted, and our safest way
Is to avoid the aim. Therefore to horse;
And let us not be dainty of leave-taking,
But shift away. There's warrant in that theft
Which steals itself when there's no mercy left.

Their precipitous flight enabled Macbeth to claim that Malcolm and Donalbain had paid Duncan's two attendants to murder their father in his sleep, and Macbeth had disposed of the grooms to prevent them from telling a different story.