  The Kilo-Degree Survey (KiDS) is an ongoing optical wide-field imaging survey
with the OmegaCAM camera at the VLT Survey Telescope, specifically designed for
measuring weak gravitational lensing by galaxies and large-scale structure.
When completed it will consist of 1350 square degrees imaged in four filters
(ugri). Here we present the fourth public data release which more than doubles
the area of sky covered by data release 3. We also include aperture-matched
ZYJHKs photometry from our partner VIKING survey on the VISTA telescope in the
photometry catalogue. We illustrate the data quality and describe the catalogue
content. Two dedicated pipelines are used for the production of the optical
data. The Astro-WISE information system is used for the production of co-added
images in the four survey bands, while a separate reduction of the r-band
images using the theli pipeline is used to provide a source catalogue suitable
for the core weak lensing science case. All data have been re-reduced for this
data release using the latest versions of the pipelines. The VIKING photometry
is obtained as forced photometry on the theli sources, using a re-reduction of
the VIKING data that starts from the VISTA pawprints. Modifications to the
pipelines with respect to earlier releases are described in detail. The
photometry is calibrated to the Gaia DR2 G band using stellar locus regression.
In this data release a total of 1006 square-degree survey tiles with stacked
ugri images are made available, accompanied by weight maps, masks, and
single-band source lists. We also provide a multi-band catalogue based on
r-band detections, including homogenized photometry and photometric redshifts,
for the whole dataset. Mean limiting magnitudes (5 sigma in a 2" aperture) are
24.23, 25.12, 25.02, 23.68 in ugri, respectively, and the mean r-band seeing is
0.70".
