This is a graphic example of an S4 preceding an S1.

Fourth heart sounds are presystolic filling sounds related to atrial systole. Because they are low in frequency, the bell of the stethoscope helps to differentiate the S4-S1 combination from a split S1. They are best heard at the tricuspid and mitral areas in patients with decreased right and left ventricular compliance respectively. When right-sided, they may augment with inspiration.