The above question requires a personal answer, based on your perception of Act IV of "Hamlet." For that reason, it is not right for me to write an answer for you, but I will show you how to write it.
To answer that question, you'll need to read Act IV of "Hamlet" and present the moments that you found most memorable and impactful. You can search for summaries of this Act, to be able to find these moments more easily.
In this case, to answer this question you should:
One of the most memorable moments of this Act is when Claudius discovers that Hamlet killed Polonius. That's because at that moment Claudius realizes how much he is in danger with Hamlet's mental instability, as this death had been thought for him.
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