<prompt> Which of the following is true about fourth heart sound S4Can be heard by the unaided earFrequency is greater than 20 HzHeard during ventricular filling phaseHeard during ventricular ejection phase  <response>: Fourth heart sound occurs due to the late rapid filling phase of the ventricle, which occurs due to atrial systole. Since its frequency is usually less than 20Hz, it is not heard by an unaided ear. 