The fourth heart sound (S4), also known as the "atrial gallop", occurs just before S1 when the atria contract to force blood into the LV.  If the LV is non-compliant and atrial contraction forces blood through the AV valves, an S4 is produced by the blood striking the LV. 

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Therefore any condition that creates a non-compliant LV will produce a S4, while any condition that creates an overly compliant LV will produce a S3(as described above).

A S4 heart sound can be an important sign of diastolic heart failure or active ischemia and is rarely a normal finding. Diastolic heart failure frequently results from severe left ventricular hypertrophy (LVH) resulting in impaired relaxation (compliance) of the LV. In this setting, a S4 is often heard. Also, if a person is actively having myocardial ischemia, adequate ATP can't be synthesized to allow for the release of myosin from actin, thus the myocardium is not able to relax and a S4 will be present.

Like S3, the S4 sound is low pitched and best heard at the apex with the patient in the left lateral decubitus position.