What is the significance of the Act II, scene iv in Macbeth?

Macbeth Act IV, scene ii is a kind of throwaway scene.  Most directors, I think, cut if from the stage play because it is only commentary: it does not advance the action of the plot.

The significance of the scene, from a literary level, is that the audience gets an outside, experienced opinion of Macbeth's heinous deed from the Old Man and a foreshadowing of Macduff as the hero of the play.

The Old Man echoes primary motifs in the play, animal imagery and the unnatural order of nature.  He says to Ross:

'Tis unnatural,
Even like the deed that's done. On Tuesday last,
A falcon, towering in her pride of place,
Was by a mousing owl hawk'd at and kill'd.

And Ross responds with:

And Duncan's horses--a thing most strange and certain--
Beauteous and swift, the minions of their race,
Turn'd wild in nature, broke their stalls, flung out,
Contending 'gainst obedience, as they would make
War with mankind.

Old Man: 'Tis said they eat each other.

After the lower-stationed Macbeth killed his superior, Duncan, the natural world has followed suit: a lower-stationed owl killed a hawk and Duncan's horses ate each other.  The subversion of nature shows the witches' predictions to be true: foul is fair and fair is foul.  The world has turned upside down: good is evil and evil is good.  The weak are eating the strong, and the strong are eating each other.  This is what happens when the King, a direct agent of God, is murdered: the natural order of society breaks down.

Later, we see that Macduff does not support Macbeth.  After Ross asks if Macduff will go to Scone to see Macbeth crowned King, Madcuff says, "No, cousin, I'll to Fife."  This subtly shows that Macduff distrusts Macbeth: he thinks he may have supported or even committed the murder.  Macduff smells an inside job and clearly does not support the new King.  He will prove to be Macbeth's nemesis and hero of the play in Act V.