Distributions of tile-by-tile data quality parameters for the KiDS DR4 data, grouped by filter, from top to bottom u, g, r and i. The light-coloured histograms represent the subset of the data that was previously released in DR1+2+3. Left: seeing. The differences between the bands reflect the observing strategy of reserving the best-seeing dark time for r-band observations. Middle column: average PSF ellipticity ⟨|e_psf|⟩, where e is defined as 1 − b/a for major/minor axis lengths a and b. Right: limiting AB magnitude (5σ in a 2″ aperture). The wider distribution of the i-band observations is a caused by variations in the moon illumination, since the i-band data were mostly taken in bright time.