ATS-4 was the second attempt at a gravity-gradient spacecraft for the ATS program. The design was much the same as ATS-2 with experiments in communications and meteorology. An additional investigation was the use of cesium propellant ion engines for stationkeeping. This time the goal was to place the satellite in synchronous orbit using the powerful Atlas-Centaur rocket.
ATS-4 was launched on August 10, 1968, but the Centaur upper stage did not ignite and the spacecraft reentered the atmosphere on October 17, 1968. The experimental ion engines were sucessfully tested in space, however.