This paper presents the fourth extension to the Very Long Baseline Array
(VLBA) Calibrator Survey, containing 258 new sources not previously
observed with very long baseline interferometry (VLBI). This survey, based on
three 24 hour VLBA observing sessions, fills remaining areas on the sky above
declination -40 degr where the calibrator density is less than one source
within a 4 degr radius disk at any given direction. The share of
these area was reduced from 4.6% to 1.9%. Source positions were derived
from astrometric analysis of group delays determined at 2.3 and 8.6 GHz
frequency bands using the Calc/Solve software package. The VCS4 catalogue of
source positions, plots of correlated flux density versus projected
baseline length, contour plots and fits files of naturally weighted CLEAN
images, as well as calibrated visibility function files are available on the
Web at http://gemini.gsfc.nasa.gov/vcs4.