When does S4 appear - early , mid or late part of Atrial Systole ?
The fourth heart sound is a low-pitched sound coincident with late diastolic filling of the ventricle due to atrial contraction. It thus occurs shortly before the first heart sound. Although it is also called the atrial sound, and its production requires an effective atrial contraction, the fourth heart sound is the result of vibrations generated within the ventricle.
During end of ventricular diastole (just before ventricular systole).