The CHaracterizing ExOPlanet Satellite (CHEOPS) will be dedicated to search for transits by means of ultrahigh precision photometry on bright stars already known to host planets. It is the first S-class mission from ESA. CHEOPS will have a nominal lifetime of 3.5 years, with launch-readiness targeted for end 2017. The purpose of this fourth CHEOPS science workshop is to bring together the community interested in the mission, providing information about its current status and preparing its scientific exploitation. A focus of the meeting will be the implementation of Guest Observation programmes for the 20% time opened on CHEOPS for the general scientific community. Participants are invited to propose contributed talks on all scientific aspects relevant to the mission, including synergies with other facilities. Ample time will be devoted to discussions.