Unraveling is the perfect word to describe what is happening to Hamlet and the others around him in act four. Is everyone familiar with a Rube Goldberg (sp?) machine? The idea is that a series of chain reactions would occur to cause one simple action. For instance, dominoes would land on a mousetrap that would cut a string so that a weight is lowered to a button... etc... and it opens a can. If the act of killing Polonius in Act 3 was Hamlet pushing over that first domino, then the entirety of act 4 is the machine going about it's business. Except, instead of opening a can, something much more macabre is about to happen. the audience is intrigued because it's interesting to watch the machine work, but horrified because they can see what's coming.
After Hamlet murdered a guy of the court, things couldn't just be ignored or covered up. A reaction started. The Queen told the King. (This seems automatic. She married him under rather suspect circumstances... I'm not surprised she goes running to him to tattle.) The King asked Hamlet where Polonius's body was. Hamlet's replies made sense, but also rang of madness. The king knew Hamlet was endangering him, so he tried to ship Hammy off to England to be neatly disposed of. (The king could not ignore that threat. Some kind of action had to be taken. What else could he have done?) Hamlet, still consumed with a desire for revenge, sends R. and G. off ahead and turns back. I guess there are several other ways they could have reacted... the King could have had Hamlet tried and punished right there-- but he was trying to avoid national strife. Hamlet could have gone on to England... but we know that wouldn't have helped him much. There are lots of mechanisms that could have been used, but the end result is the same. The can gets opened. The characters can't see (in fact, it's difficult to imagine) a way to stop this chain now that it has begun.
Ophelia and Laertes have the kind of reactions one could expect from them. Ophelia goes mad with grief (typical shakespearian treatment of a female?? hmm...) and Laertes goes for revenge. The king manages to turn Laertes anger to Hamlet, the one who actually killed Polonius, and they make up their little plot to kill Hamlet and set it up so that "His means of death, his obscure funeral-- no trophy, sword, nor hatchment over his bones..." Ironic that they're talking about themselves, too. Because all of the males in this play seem controlled by anger and the need to get revenge, it's just another automatic action in the chain leading to the final opening of the can.