I think that Ophelia does in fact go mad within act 4. Considering all that Ophelia has gone through it only makes sense that her fragile mind has shattered. There are a few factors that stacked up to cause Ophelia to go crazy, the first I think is when Ophelia's father and brother tell her that she has to stop seeing Hamlet for her own sake. Laertes says "Forward, not permanent, sweet, not lasting, the perfume and suppliance of a minute, no more." (1,3,9-11) Ophelia loved Hamlet but because she is very obedient she did what she was told and cut off her relationship with Hamlet, which was depressing for both Hamlet and Ophelia. Another factor that lead to Ophelia going crazy is the death of her father. I think for anyone the death of someone who you loved very much and have looked up to for your entire life would make anyone really depressed. Not only did her father die but he was killed by Hamlet who she still has feelings for, so that must have been very traumatic, not only having her father die but him being killed by someone she loves. This is a lot for anyone to handle and to make things worse Ophelia brother is still in France so she has to deal with all of this by herself. All of that happening all at once with no one to help you cope with the events that just occurred would overwhelm anyone. Ophelia is ultimately overwhelmed with events which drives her to the edge of sanity. So
by the time her brother gets back and finds out what happened Ophelia has lost her mind at this point. We see this as he tries to talk to her but nothing but nonsense comes out of her mouth. Even when she is asked simple questions her answers make no sense, her response to when the king asks her how she is "Well, good 'ild you. They say the owl was a baker's daughter. Lord, we know what we are, but know not what we may be. God be at your table." (4,5,44-46) As we see the first thing she says answers the king's question but the rest of her response has no relevance to what anyone was saying and it does not make sense. Even the king says that she has gone crazy due to deep grief "O, this is the
poison of deep grief!" (4,5,77) 
     As for what this symbolizes about rest of the kingdom is that the
kingdom is falling apart. Since the play has started everyone's sanity
seems to be diminishing, even a person has died since the
beginning. At the beginning Hamlet was still sane, but just depressed, as for Ophelia she was a perfectly normal girl with no issues at all. Though at this point in the play Hamlet has gone insane and has even gained the title of murderer, Ophelia has suffered the failure of a relationship as well as the death of her father. Even Claudius has changed for the worse since the beginning of the play, at the start of the play he was just a regular king, besides the fact that he has supposedly killed a person, but at this point in the play his mind is filled only with different ways to kill Hamlet. At this point in the play the kingdom has fallen apart because there is no peace within it. Everyone has turned on each other, with the king and Claudius wanting to kill Hamlet and Hamlet wanting to kill Claudius. There is no hope for the kingdom to return to the way it used to be, everyone one is much too set on the evil intentions to try and repair their relationships with each other opposed to trying to end them.