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. .
When a fourth heart sound is heard in young patients with aortic outflow obstruction, it usually indicates that the lesion is severe, as severe obstruction results in ventricular hypertrophy and reduced ventricular compliance. In patients over the age of 40, this correlation is poor, as other causes may contribute, such as coronary disease or hypertension. In the elderly patient, it is not clear if an S4 invariably implies heart disease.