In act 5, scene 4 of Macbeth, what is Malcolm's plan?

In Act 5, scene 4, of Shakespeare's Macbeth, Malcolm (Duncan's oldest son, named to be the dead king's successor) has decided to attack Macbeth at Inverness. Upon defeating the tyrant who murdered his father, Malcolm plans to take back the throne and rid Scotland of its murderous king. In order to accomplish this, Malcolm gives strategic instructions to his men.

As they approach the castle, Malcolm and his army come to a wooded area that Menteith identifies as Birnam Wood. In order to camouflage their numbers and surprise Macbeth (covering the sizable force moving to engage him), Malcolm tells his soldiers to cut branches from the trees and use them to shield themselves. (It is in this way that Birnam Wood appears to move.)

MALCOLM:

Let every soldier hew him down a bough,
And bear't before him: thereby shall we shadow
The numbers of our host, and make discovery
Err in report of us. (V.iv.6-9)

Malcolm's plan is to show Macbeth's lookouts a false front so they will be unable to ascertain the true number of soldiers preparing to attack. The lookouts will provide Macbeth with a number smaller than the actual army advancing on Inverness. By the time Macbeth realizes the mistake, it will be too late for him to defend against such vast numbers, especially because (as Malcolm reports) aristocrats and commoners (everyone with a heart) have fled, abandoning Macbeth to his fate. (15-18)

What tactical strategy does Malcolm use in Act 5, Scene 4 of Macbeth?

Malcolm and King Edward's soldiers have encamped close to Birnam Wood not far from Macbeth's castle. Malcolm gives the following instruction to the troops:

Let every soldier hew him down a bough
And bear't before him: thereby shall we shadow
The numbers of our host and make discovery
Err in report of us.

He is, in effect, ordering every soldier to cut down a tree branch and hold it before him. He states that, in doing this, they will disguise their numbers and, therefore increase the probability that the true number of soldiers will be reported incorrectly. This is a clever camouflage tactic because Macbeth has no idea how many soldiers King Edward of England has provided Malcolm with in his effort to overthrow him and retake Scotland.

If one were to view, from a distance, the soldiers thus disguised marching towards Macbeth's castle at Dunsinane, it would seem as if the forest itself was on the move. When Macbeth is informed that the forest is moving, he reacts angrily:

Messenger
As I did stand my watch upon the hill,
I look'd toward Birnam, and anon, methought,
The wood began to move.

Macbeth
If thou speak'st false,
Upon the next tree shalt thou hang alive,
Till famine cling thee: if thy speech be sooth,
I care not if thou dost for me as much.
I pull in resolution, and begin
To doubt the equivocation of the fiend
That lies like truth: 'Fear not, till Birnam wood
Do come to Dunsinane:' and now a wood
Comes toward Dunsinane.

Macbeth expresses his despair because he now realizes that the witches' prediction that

Macbeth shall never vanquish'd be until
Great Birnam wood to high Dunsinane hill
Shall come against him

was, in fact, an equivocal declaration that he believed in a literal sense. He believed the witches meant that the forest would literally uproot itself and march towards his castle—an impossibility. At the time he saw these as "sweet bodements." He now expresses doubt about the veracity of the witches' prognostications and finally realizes that they were deliberate lies meant to deceive him.