Apex: Fourth Heart Sound (S4)
The fourth heart sound is a low frequency sound heard just before the first heart sound. It is most commonly known as S4.
The S4 is a diastolic sound which occurs during the late diastolic filling phase at the time when the atria contract. When the ventricles have a decreased compliance or are receiving an increased diastolic volume, they generate a low frequency vibration, the S4.
The S4 of left ventricular origin is best heard at the apex with the patient supine, or in the left lateral recumbent position. Its causes include severe hypertension, aortic stenosis, cardiomyopathies, and left ventricular myocardial infarctions.
The sound you heard was an S4 recorded from the apex of a 60 year old male with cardiac disease.