Compare Hamlet's soliloquy at the end of Act IV, Scene 4, with the one in Act II, Scene 2. Based on his sentiments and the themes in these two...

In Act 2, Scene 2, Hamlet passes severe judgment on himself. His harsh criticism stems from what he believes to be cowardice. Hamlet made a solemn promise to his father's ghost that he would avenge his father's death at the earliest opportunity. Some time has already passed, but Hamlet has failed to fulfill his pledge.

Hamlet uses harsh terms in his self-criticism, comparing himself to an actor who can so easily appear sorrowful and anxious by merely thinking about something of no real importance to him:

...this player here,
But in a fiction, in a dream of passion,
Could force his soul so to his own conceit
That from her working all his visage wann'd,
Tears in his eyes, distraction in's aspect,
A broken voice, and his whole function suiting
With forms to his conceit? and all for nothing!

Hamlet cannot summon up enough courage to take revenge, though.

...Yet I,
A dull and muddy-mettled rascal, peak,
Like John-a-dreams, unpregnant of my cause,
And can say nothing; no, not for a king,
Upon whose property and most dear life
A damn'd defeat was made.

Hamlet thinks he is not driven enough and resents his ineptitude at this characteristic. He acknowledges he has plenty of reasons to spur him on but lacks the courage to act. Hamlet questions his mettle and, through a series of rhetorical questions, damns himself as his own worst critic. He concedes that he should accept this cruel condemnation, however, since he is a coward who lacks the will to enact retribution. 

Hamlet passionately expresses his contempt for Claudius:

With this slave's offal: bloody, bawdy villain!
Remorseless, treacherous, lecherous, kindless villain!

His apostrophic cry to vengeance is an indication of the torment he is suffering and heightens his sense of incompetence. He calls himself a fool and repeats that he has ample reason to act, stating that he has both malice and goodness to prompt him. Hamlet compares himself to a whore who expresses her anger by swearing and ranting and raving, and to a lowly servant who curses her resentment.

At the end of his impassioned monologue, Hamlet decides to use the actors in a play which, he believes, should provoke a reaction from Claudius that proves his guilt. Hamlet believes the king's response will provide better grounds for his revenge than mere words would:

the play's the thing
Wherein I'll catch the conscience of the king.

Although Hamlet is still quite critical of himself in Act IV, Scene 4, he is less harsh. He repeats that he has more than enough reason to enact his vengeance, but alludes to his lack of intent. He compares himself to a beast that only sleeps and feeds. He admits that he is not quite sure why he has not acted on his vow yet. He contemplates whether it is because he is mostly a coward or because he seeks and thinks too much about a perfect moment in which to commit the deed.

At this point, Hamlet concedes that he has the means, strength and will to avenge his father's death. In this sense, Hamlet's conviction is far greater than it was in his earlier soliloquy. Hamlet compares his situation with that of prince Fortinbras, who came to claim a piece of useless Polish soil as a matter of honor.

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.

In this comparison, Hamlet states that greatness is not determined by the size of an issue that one challenges, but by one's moral purpose—there is virtue in fighting for something as a matter of honor. In this, Hamlet sees himself as having greater purpose. Why, then, should he let "it sleep?" Hamlet feels ashamed that twenty thousand men will die fighting for a plot of land which is not large enough to bury them all. In a determined declaration, Hamlet states,

O, from this time forth,
My thoughts be bloody, or be nothing worth!

There is a clear contrast in Hamlet's opinion of himself between the two monologues. In the first, his self-denunciation is powerful and he lacks a clear resolve. It appears his only desire is to create a scenario in which to confirm his stepfather's guilt. Although he is still mildly critical of himself in the second monologue, Hamlet appears to be much more positive and to have finally made up his mind to kill Claudius.