Character from Act Four

Hamlet in Act four truly surprised me. He has finally lost it in my opinion. I know that he pretended to be "insane" but why would he jump to the conclusion that Claudius is behind the arras? He has gone from someone of potential in the story to a helpless soul. I don't know what will happen in England but I'm guessing that his uncle wants to do away with him now that Hamlet knows that Rosencrantz and Guildenstern are his spies and that Hamlet attempted to kill him when killing Polonius. I hope Hamlet can avenge his father's death and remain alive in England but there is nothing in this Act to reassure me of his hero like qualities I was exposed to in the first act two acts of the play.