I think it can take a full day to recover from a 4 hour exam, whether it be practice or the real deal. I think a study plan that includes 1 to 2 hour sessions most of the time, with the occasional 4 hour burst, i.e. for practice tests, is perfect. Its like training for a marathon. You rarely will run the full 26.2 miles in training, but will try to do about 20 miles, and the excitement and adrenaline of race/test day will carry you through to peak performance.