This program is undertaken in order to construct and field-demonstrate 'EXPLORER', a modular, remotely controllable, self-powered, untethered robot system for the inspection of live gas distribution 150 mm (6- inch) to 200 mm (8-inch) diameter mains. The modular design of the system allows it to accommodate various components intended to accomplish different inspection, repair, sample retrieval, and other in-pipe tasks. The prototype system being built under this project will include all the basic modules needed, i.e. the locomotor, power storage, wireless communication, and camera. The camera, a solid-state fisheye-type, is used to transmit real-time video to the operator that allows for the live inspection of gas distribution pipes. The system under development significantly advances the state of the art in inspection systems for gas distribution mains, which presently consist of tethered systems of limited range (about 500 ft form the point of launch) and limited inspection views. Also current inspection systems have no ability to incorporate additional modules to expand their functionality. This development program is a joint effort among the Northeast Gas Association (formerly New York Gas Group), the Jet Propulsion Laboratory (JPL), the Johnson Space Center (JSC), Carnegie Mellon University's (CMU) National Robotics Engineering Consortium (NREC), and the US Department of Energy (DOE) through the National Energy Technology Laboratory (NETL) The present report summarizes the accomplishments of the project during its fourth six-month period. The project has in general achieved its goals for this period as outlined in the report. The fabrication of the prototype is complete and is now been tested in the laboratory mainly focusing on endurance testing and testing of launching procedures. Testing of the prototype in the lab is expected to be completed by Fall 2003, to be followed by two field demonstrations in Winter 2003-2004.