There are two situations discussed in Act IV, scene iii -- the public and private. The private situation discussed is the murder of Macduff's family because of the charge of treason that Macbeth has laid upon Macduff's head. To which charge, Macduff says simply, "I am not treacherous." And Malcolm answers, "But Macbeth is."
This leads to the discussion between the two about the public situation, a situation totally controlled by Macbeth. The two lament the state of affairs in Scotland, recognizing that the murder of Macduff's family might well be the straw that breaks the camel's back, not just because it is a heinous crime against Macduff and his wife and child, but because it is the act of a tyrant. Macduff says:
Bleed, bleed, poor country!
Great tyranny, lay thou thy basis sure,
For goodness dare not check thee. Wear thou thy wrongs,
The title is affeer'd! Fare thee well, lord,
I would not be the villain that thou think'st
For the whole space that's in the tyrant's grasp. . .
I think our country sinks beneath the yoke:
It weeps, it bleeds. And each new day a gash
Is added to her wounds. . . .
When I shall tread upon the tyrant's head,
Or wear it on my sword, yet my poor country
Shall have more vices than it had before,
More suffer and more sundry ways than ever,
By him that shall succeed.
And it is Malcolm who will succeed Macbeth. The conversation of this scene finally bends towards this. And though, Malcolm pleads a modest inadequacy to assuming the throne of Scotland, by the end of the scene, Malcolm and Macduff have agreed to ally with England to win back their Scotland from the tyrannical hold of Macbeth.