The fourth heart sound is caused by:

Atrial Contraction Sound (Fourth Heart Sound).  An  atrial  heart  sound  can  sometimes  be  recorded  in the phonocardiogram, but it can almost never be heard with a stethoscope because of its weakness and very low frequency—usually  20  cycles/sec  or  less.  This  sound occurs  when  the  atria  contract,  and  presumably,  it  is caused by the inrush of blood into the ventricles, which initiates vibrations  similar to those of the third heart sound. A fourth heart sound is common in persons who derive  benefit from atrial contraction for ventricular filling as a result of decreased ventricular wall compliance and increased resistance to ventricular filling. For example, a fourth heart sound is often heard in older patients with left ventricular hypertrophy. Reference: GUYTON AND HALL TEXTBOOK OF MEDICAL PHYSIOLOGY, THIRTEENTH EDITION(INTERNATIONAL EDITION ) page no 284