We present the first public data release (DR1) of the KMTNet Synoptic Survey of Southern Sky (KS4). This deep, wide-field imaging survey covers a southern footprint of -85{deg}<DEC<-28.8{deg} in the B, V, R, and I bands using a network of three 1.6-m telescopes. Although primarily designed to secure reference imaging for gravitational wave counterpart identification, DR1 delivers science-ready data for ~4000deg2 to enable a broad range of astrophysical research. The release includes deep co-added images reaching median 5{sigma} depths of 22.0-23.5 AB mag. It is accompanied by two source catalogs containing over 200 million sources with SNR>5: an I-band-selected forced-photometry catalog optimized for consistent colors, and a band-merged catalog offering enhanced completeness. Validation demonstrates robust data quality, characterized by mean astrometric offsets of +0.054+/-0.129arcsec in RA and -0.015+/-0.120arcsec in DEC relative to Gaia DR3. Photometric uniformity for point sources is maintained within +/-0.03mag relative to Gaia XP for 97.5-99.8% of the footprint across all four bands. A key advantage of KS4 is its uniform and contiguous spatial coverage. It extends to fainter magnitudes than other uniform surveys while filling irregular gaps in existing deep datasets.