Shouldn't this be "Tu vas parteger le gâteau en quatre morceaux?" Otherwise, the question-sentence should have been "You are going to divide the cake into four," rather than "You are going to divide the cake into four parts." Right?
Oh this does not make sense if I do not unless I state that I received the only correct answer as "Tu vas parteger le gâteaux en quatre," when the question was "You are going to divide the cake into four parts."