  We present the fourth data release (DR4) of the SkyMapper Southern Survey
(SMSS), the last major step in our hemispheric survey with six optical filters:
u, v, g, r, i, z. SMSS DR4 covers 26,000 sq.deg from over 400,000 images
acquired by the 1.3m SkyMapper telescope between 2014-03 and 2021-09. The
6-band sky coverage extends from the South Celestial Pole to Dec = +16deg, with
some images reaching Dec ~ +28deg. In contrast to previous DRs, we include all
good-quality images from the facility taken during that time span, not only
those explicitly taken for the public Survey. From the image dataset, we
produce a catalogue of nearly 13 billion detections made from ~700 million
unique astrophysical objects. The typical 10sigma depths for each field range
between 18.5 and 20.5 mag, depending on the filter, but certain sky regions
include longer exposures that reach as deep as 22 mag in some filters. As with
previous SMSS catalogues, we have cross-matched with a host of other imaging
and spectroscopic datasets to facilitate additional science outcomes. SMSS DR4
is now available to the worldwide astronomical community.
