Optical detection of gravitational wave source, GW170817 opened era of multi-messenger astronomy. Next observation run (O4) of gravitational wave detectors will start late of 2021 with higher sensitivity. To identify optical counterpart of gravitational wave source, we need reference images fully covering all sky with ~23 mag depth. Many wide-area sky surveys have performed in optical so far and cover a lot of area of the sky. However, southern sky is not fully covered or shallowly covered than northern hemisphere. We started KMTNet Synoptic Survey of Southern Sky which will image ~7000 deg2 in BVRI bands using wide-field telescopes of KMTNet. The survey will take images with ~2 arcsec seeing and 23.0 mag depth. Not only for the main purpose of reference image, KS4 data will also be used to study high redshift quasars and galaxy clusters with multi-wavelength data.