What specifically has happened in scene iv to prompt the thoughts hamlet expresses in his soliloquy?
hamlet killed polonius and realized that revenge is not the proper way to operate.
he found out that claudius sent rosencrantz and guildenstern to have hamlet killed. hamlet now knows that he must kill claudius or
be killed himself.
hamlet has seen that both norway and poland are willing to lose lives and money over a piece of land and that he has a much
greater cause.

Hamlet has seen that both Norway and Poland are willing to lose lives and money over a piece of land and that he has a much  greater cause.

Explanation:

Shakespeare's Hamlet is a play about revenge that Hamlet seeks on his uncle, Claudius. Hamlet has promised the ghost of his father that he will revenge him, but he procrastinates in doing so.

In Act IV, Scene IV,  Hamlet is on his way to the ship bound for England, together with Rosencrantz and Guildenstern, when they are informed by the captain that the Norwegian army is preparing for a battle with the Poles.  Hamlet is told that the two armies will go to the battle over ''a little patch of land / That hath in it no profit but the name.''

The fact that the two armies are ready to lose so many lives for a piece of land which is, practically, worthless, has a great impact on Hamlet.

Hamlet's revenge plan has a real purpose, but he still hesitates and does not take any action. Angry at himself for waiting for so long, Hamlet promises that from this moment on, ''My thoughts be bloody, or be nothing worth!''

c.to compare several aspects of the tragedy to other worldwide tragedies

fond of fighting is the answer