Before I get into the words, I would just like to share one particular reason why I love this speech and why I think it's important. As you may or may not know, there are at least two versions Hamlet: the quarto (single copy) of 1603 and the folio (complete works) version of 1623. The quarto version, as we know it, is believed to have been transcribed by some of the actors in the play and is therefore probably based on what they remember from their performance, and is thus shorter. The folio version, though printed after Shakespeare's death, is believed to be the full version as Shakespeare intended.
The debate as to which is the "true" Hamlet continues to this day and some productions (and even printed editions of the play) are produced according to the shorter, quarto version.
Act IV.4 is one such scene that belongs to the folio version, but not the quarto version. Therefore, sometimes, you'll see a stage production or read a version of the play that is missing this very important scene. Personally, I think it's unfortunate to strike this scene from the record.
Why I particularly love Hamlet's big speech in this scene is because it really speaks to the true genius that was Shakespeare with regard to the way he constructs a play. If you think about it, Act II.2 and Act IV.4 reflect each other, which I think makes total sense or is at least the result of a sublime coincidence. Multiply II.2 by itself and the product is IV.4, with perhaps even double the intensity, too. The two scenes, then, hold the mirror up to each other (in Act III.2, Hamlet speaks of holding the mirror to nature; I believe that these two speeches both hold the mirror up to nature, with regard to the way Hamlet thinks and continues to ponder similar issues/problems throughout the play, but they also hold the mirror up to each other).
The "What a piece of work is a man" speech to R & G in Act II.2, parallels the Act IV.4 speech, when Hamlet reacts to seeing Fortinbras gathering his power, which is very similar to the way he reacts at the end of II.2 to the player’s recitation of the Hecuba saga. It’s as if, then, II.2 is seeing a double image of itself. With regard to the theme of the IV.4 speech, Hamlet, once again, discourses on the nature of man, as he sees it. The recurrence of this thought, the swirl of this idea--as a song that plays in one’s head over and over and over again and she can't get it out--is about as true to nature/human nature as it gets.
Here's a look at some of those words from IV.4:
...What is a man
If his chief good and market of his time
Be but to sleep and feed? A beast, no more...
Consider, "A beast, no more." We can read this as, "A beast, and nothing more (if all man does is eat and sleep); that's it." Or perhaps Hamlet, is introducing his next thought:
Sure, He that made us with such large discourse,
Looking before and after, gave us not
That capability and godlike reason
To fust in us unused...
A beast, no more. Perhaps Hamlet is saying "A beast, but not any more; now's the time to plan and act... once and for all."
Remember all the things that Hamlet says about man in II.2, only to go on to say that man delights him not. Perhaps man delights him not, because man does not use his gift of reason properly. "To fust in us unused." The creator did not give us this gift, just to have us allow it to go unused and rot inside us--'tis a wasted gift, then. Hamlet will once and for all put the gift of reason to good use by putting it into action.
This also supports his advice to the players in III.2, "Suit the action to the word, the word to the action."