Fourth heart sound 2017-12-17T16:03:22Z Atrial gallop The fourth heart sound (S4) is a low-pitched sound that occurs just before the first heart sound (S1) when the atria contract to force blood into the left ventricle, that is, coincident with late diastolic filling of the ventricle due to atrial contraction. S4 occurs in late diastole, is associated with atrial contraction, and coincides with the P wave on the EKG. Presystolic gallop S4 Gallop rhythm