  We present both the technical overview and main science drivers of the fourth
phase of the Optical Gravitational Lensing Experiment (hereafter OGLE-IV).
OGLE-IV is currently one of the largest sky variability surveys worldwide,
targeting the densest stellar regions of the sky. The survey covers over 3000
square degrees in the sky and monitors regularly over a billion sources.
  The main targets include the inner Galactic Bulge and the Magellanic System.
Their photometry spans the range of $12<I<21$ mag and $13<I<21.7$ mag,
respectively. Supplementary shallower Galaxy Variability Survey covers the
extended Galactic bulge and 2/3 of the whole Galactic disk within the magnitude
range of $10<I<19$ mag. All OGLE-IV surveys provide photometry with
milli-magnitude accuracy at the bright end. The cadence of observations varies
from 19-60 minutes in the inner Galactic bulge to 1-3 days in the remaining
Galactic bulge fields, Magellanic System and the Galactic disk.
  OGLE-IV provides the astronomical community with a number of real time
services. The Early Warning System (EWS) contains information on two thousand
gravitational microlensing events being discovered in real time annually, the
OGLE Transient Detection System (OTDS) delivers over 200 supernovae a year. We
also provide the real time photometry of unpredictable variables such as
optical counterparts to the X-ray sources and R CrB stars.
  Hundreds of thousands new variable stars have already been discovered and
classified by the OGLE survey. The number of new detections will be at least
doubled during the current OGLE-IV phase. The survey was designed and optimized
primarily to conduct the second generation microlensing survey for exoplanets.
It has already contributed significantly to the increase of the discovery rate
of microlensing exoplanets and free-floating planets.
