Act IV, Scene 4

Despite its containing one of Hamlet's soliloquies, it's not uncommon to see this scene excised from any given adaptation. Obviously, those that remove Fortinbras entirely won't be able to justify Hamlet crossing paths with his army on his way to exile in England. And a director might legitimately ask if their adaptation needs another moment where Hamlet decides that NOW he must act. In many ways, this is a mirror of "O, what a rogue and peasant slave am I" (Act II, scene 2). There, he was shamed by the First Player whose ability to express a false passion was greater than his to express a true ambition. Here, Hamlet is shamed by Norway and Poland, who attack or defend a worthless patch of soil with more conviction and willful action than he went after his revenge. Fortinbras is, after all, Hamlet's opposite number, a princely man of action, perhaps far more like Hamlet Sr. than his scholarly son. In him, Hamlet finds a role model, and perhaps enough of the qualities he would like to see in himself to give the Norwegian prince his vote of confidence at the play's conclusion. But let's look at Shakespeare's text (in italics; my comments interrupting in normal script) before heading off into those adaptations that included the scene.

SCENE IV. A plain in Denmark.

Enter FORTINBRAS, a Captain, and Soldiers, marching
PRINCE FORTINBRAS: Go, captain, from me greet the Danish king;
Tell him that, by his licence, Fortinbras
Craves the conveyance of a promised march
Over his kingdom. You know the rendezvous.
If that his majesty would aught with us,
We shall express our duty in his eye;

There is perhaps an early clue to Fortinbras' treachery. Expressing his duty "in his eye" might be something done for appearances only, since his plan is to invade Denmark after the invasion of Poland. This is Fortinbras' first appearance on stage, and it doesn't tell us much, only recaps the deal he has with Claudius as per the wedding banquet scene. But that's the point. Fortinbras is a plain-spoken man who deals in facts and not abstractions. A doer, not a thinker. At least, not beyond the tactics of the day.

And let him know so.
CAPTAIN: I will do't, my lord.
PRINCE FORTINBRAS: Go softly on.

Exeunt FORTINBRAS and Soldiers
Enter HAMLET, ROSENCRANTZ, GUILDENSTERN, and others

HAMLET: Good sir, whose powers are these?
CAPTAIN: They are of Norway, sir.
HAMLET: How purposed, sir, I pray you?
CAPTAIN: Against some part of Poland.
HAMLET: Who commands them, sir?
CAPTAIN: The nephews to old Norway, Fortinbras.
HAMLET: Goes it against the main of Poland, sir,
Or for some frontier?

I'm not sure I've ever seen it played that way, but Hamlet's questions could indicate a certain suspicion as to Fortinbras' intent. After all, he's just discovered a foreign army on Danish soil. The Captain's candor and sincerity (he is like his master, just as Hamlet is a product of his own deceptive Court) relieve him of his suspicions, however, as Shakespeare offers a tidy satire on the absurdity of war from the soldiers' point of view.

CAPTAIN: Truly to speak, and with no addition,
We go to gain a little patch of ground
That hath in it no profit but the name.
To pay five ducats, five, I would not farm it;
Nor will it yield to Norway or the Pole
A ranker rate, should it be sold in fee.
HAMLET: Why, then the Polack never will defend it.
CAPTAIN: Yes, it is already garrison'd.
HAMLET: Two thousand souls and twenty thousand ducats
Will not debate the question of this straw:
This is the imposthume of much wealth and peace,
That inward breaks, and shows no cause without
Why the man dies. I humbly thank you, sir.
CAPTAIN: God be wi' you, sir.

Exit

ROSENCRANTZ: Wilt please you go, my lord?
HAMLET: I'll be with you straight go a little before.

Exeunt all except HAMLET

How all occasions do inform against me,
And spur my dull revenge! What is a man,
If his chief good and market of his time
Be but to sleep and feed? a beast, no more.

Hamlet invokes his earlier "quintessence of dust" speech, but where earlier he condemned humanity, here he condemns how men abuse other men and render their existence meaningless. He's turned a corner. Where before, he saw humanity as a futile state of being, now he accepts that humanity is not necessarily futile, but one must overthrow the chains of that futility. Relative to the cannon fodder before him, he has power enough to make his life (and death) count, and resolves to do so. He even admits God's gift of free will and thus rejects predestination and nihilism:

Sure, he that made us with such large discourse,
Looking before and after, gave us not
That capability and god-like reason
To fust in us unused. Now, whether it be
Bestial oblivion, or some craven scruple
Of thinking too precisely on the event,
A thought which, quarter'd, hath but one part wisdom
And ever three parts coward, I do not know
Why yet I live to say 'This thing's to do;'

Hamlet counts himself lucky to still be alive, but can't believe he still hasn't done what he set out to do. Hamlet looks into himself and does not know exactly why he's delayed. Blaming "bestial oblivion", i.e. having forgotten his mission and Claudius' deeds, may not seem possible on the surface of it, but his confusion between blaming Claudius and blaming Gertrude might be what he's referring to. The question he asks himself is whether emotion (the realm of the unthinking beast) or reason (thinking himself into inaction) was to blame, not that it makes a difference.

Sith I have cause and will and strength and means
To do't. Examples gross as earth exhort me:
Witness this army of such mass and charge
Led by a delicate and tender prince,
Whose spirit with divine ambition puff'd
Makes mouths at the invisible event,
Exposing what is mortal and unsure
To all that fortune, death and danger dare,
Even for an egg-shell. Rightly to be great
Is not to stir without great argument,
But greatly to find quarrel in a straw
When honour's at the stake. How stand I then,

His admiration for Fortinbras takes root here, admiration for his ability to take risks and action. He hasn't met Fortinbras, of course, so one might suspect Hamlet seeing a bit of himself in him. Certainly, he's the more tender and delicate prince. Psychologically, Hamlet is turning himself into a Fortinbras, readying himself for his eventual return.

That have a father kill'd, a mother stain'd,
Excitements of my reason and my blood,
And let all sleep? while, to my shame, I see
The imminent death of twenty thousand men,
That, for a fantasy and trick of fame,
Go to their graves like beds, fight for a plot
Whereon the numbers cannot try the cause,
Which is not tomb enough and continent
To hide the slain? O, from this time forth,
My thoughts be bloody, or be nothing worth!

Exit

Hamlet's tried to convince himself to take action before, but this is the one that takes. By the time we see him again, he'll have arranged two deaths and will be ready to kill more. There really is no going back, though ironically, he's leaving Denmark and will have to literally go back.