S4, the fourth heart sound in the cardiac cycle. It occurs late in diastole on contraction of the atria. Rarely heard in normal subjects, it indicates an abnormally increased resistance to ventricular filling, as in hypertensive cardiovascular disease, coronary artery disease, cardiomyopathy, and aortic stenosis. A left-sided fourth heart sound, it may be heard with the stethoscope’s bell at the apex of the heart during expiration. Also called atrial gallop, physiological fourth heart sound.
