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We present both the technical overview and main science drivers of the fourthphase 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 3000square 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 theextended Galactic bulge and 2/3 of the whole Galactic disk within the magnituderange of $10<I<19$ mag. All OGLE-IV surveys provide photometry withmilli-magnitude accuracy at the bright end. The cadence of observations variesfrom 19-60 minutes in the inner Galactic bulge to 1-3 days in the remainingGalactic bulge fields, Magellanic System and the Galactic disk.  OGLE-IV provides the astronomical community with a number of real timeservices. The Early Warning System (EWS) contains information on two thousandgravitational microlensing events being discovered in real time annually, theOGLE Transient Detection System (OTDS) delivers over 200 supernovae a year. Wealso provide the real time photometry of unpredictable variables such asoptical counterparts to the X-ray sources and R CrB stars.  Hundreds of thousands new variable stars have already been discovered andclassified by the OGLE survey. The number of new detections will be at leastdoubled during the current OGLE-IV phase. The survey was designed and optimizedprimarily to conduct the second generation microlensing survey for exoplanets.It has already contributed significantly to the increase of the discovery rateof microlensing exoplanets and free-floating planets.

OGLE-IV: Fourth Phase of the Optical Gravitational Lensing Experiment