Why might you agree with this statement on the basis of act four? A past exam question, in part reads: "The action of the play begins to break down after act three."
After his killing of Polonius at the end of Act III, Hamlet is no longer a hero in the conventional dramatic sense. So long as he was planning to avenge the murder of his father, all was well and we were very much on his side. But after having dispatched poor old Polonius, it's difficult for us to feel that Hamlet enjoys much in the way of moral superiority over Claudius. Polonius's murder may well have been accidental, but Hamlet shows no contrition whatsoever in depriving Ophelia and Laertes of a father just as Claudius deprived Hamlet of his.
One could say that the action does indeed break down in Act IV in that the focus of the play becomes more firmly fixed on Hamlet's increasingly tumultuous mental state than on his desire to wreak revenge. There is still plenty of action to be sure but much of it appears divorced from Hamlet's initial plan of action. The exploration of character becomes ever more important than action in Act IV. It is this feature more than any other that gives it something of a lopsided feel.
Ah, good question. You could see the action of the play as breaking down after Act III because it changes in nature. The first three acts are tightly focused, and tension is building. After that, though—and I'd marked the turning point at the place when Hamlet kills Polonius—the focus of the play seems to shift. Earlier on, Hamlet was testing the ghost's message, seeking evidence of guilt, and trying to kill Claudius. After that, we've got Hamlet sent away, Ophelia going crazy, the graveyard scene, etc. Even the simple fact that there are seven scenes to Act IV shows a kind of structural imbalance.