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Fourth heart sound the Atrial sound

S4 signifies systolic overload of the ventricles.
It occurs during the last rapid filling phase of cardiac cycle (in ventricular diastole)
Which also coincides with the atrial systole.
Also called as presystolic or atrial gallop.
Physiology of S4
It is a low pitched sound in presystole heard just before 2nd heart sound.
Mechanism of S4
Fourth heart sound is produced due to forcible atrial contraction to fill the non-complaint ventricle in ventricular hypertrophy or infarct, this will produce stretch on myocardium which inturn produce tense chordae and papillary muscle to produce S4.
The fourth heart sound is due to increased left ventricular/right ventricular end-diastolic pressure
S4 is produced by a rapid emptying of the atrium into ventricle.
S4 is recordable, but inaudible in children adults.