To my mind, this soliloquy runs the risk of being overlooked in terms of what it reveals about Hamlet at this stage of the play, so well done for focussing on it! In Act IV scene 4 Hamlet meets Fortinbras's army, which is heading across Denmark on its way to fight Poland. After the army has left, Hamlet, in his final soliloquy of the play, examines the action of Fortinbras and compares it to his own. This soliloquy bears much resemblance to when Hamlet compared himself with the actor in Act II scene 2, as both comparisons leave Hamlet feeling ashamed. He resolves to have more bloody thoughts from now on.
It is well worth analysing this speech in detail, however, because in this soliloquy Shakespeare seems to give a penetrating insight into the processes of Hamlet's tortured thinking. Again, he moves from self-disgust to a decision to act. His final words sound determined:
Oh from this time forth,
My thoughts be bloody, or be nothing worth.
And yet this is shown to be ironic. As so often, Hamlet's actions contradict his words. His thoughts appear to move logically towards the bloodthirsty conclusion, and yet the speech is a mass of contradictions. Shakespeare is showing us that what someone says is not always what they believe.
Consider this part of his soliloquy:
I do not know
Why yet I live to say this thing's to do,
Sith I have cause, and will, and strength, and means
Hamlet seems to be not being honest with himself. In fear of divine punishment from a Christian God, Hamlet definitely does not possess the "will" to take revenge.
Hamlet proceeds to contradict himself explicitly:
Examples gross as earth exhort me.
This is clearly illogical. Things that are "gross" or foul cannot encourage you to to do anything except the opposite. Hamlet was sickened by what Fortinbras's army was going off to do. Now, he feels their behaviour impelling him to do something similarly "honourable", but honourable from the pagan perspective. Hamlet then re-writes what he has just seen, changing his view on Fortinbras and his conquest, trying to project himself into his shoes and play the role his father would have wished him to play. Hamlet somehow reasons himself into seeing the world upside-down, and yet, in this topsy-turvy view, there are hints of something "rotten" in it - he admits that "fame" is "a fantasy and a trick", even as he states his defiant last couplet and goes off supposedly full of revenge.
This soliloquy, then, is highly interesting because it shows the contradictory state of Hamlet and his mind and, most importantly, his attitude towards taking the revenge his father wishes him to take.