The Department of Defense's Advanced Research Projects Agency (ARPA) had initially planned two satellites for the purposes of studying the Van Allen radiation belts and the effects of nuclear explosions upon these belts (and the Earth's magnetosphere in general), however Explorer 4 was the only such satellite launched as the other, Explorer 5, suffered launch failure.

Explorer 4 was a cylindrically shaped satellite instrumented to make the first detailed measurements of charged particles (protons and electrons) trapped in the terrestrial radiation belts.

The launch vehicle was a Juno I, a variant of the three-stage Jupiter-C with an added fourth propulsive stage, which in this case was the Explorer 4.

Soon after orbit insertion, the spacecraft developed an end-over-end tumbling motion with a period of about 6 seconds, which affected the measurements and signal level throughout the mission.

Explorer 4 was in orbit and operational during the three Project Argus launches 27 August 1958 to 6 September 1958, part of the mission objective was to observe the effects of these high-altitude A-bomb detonations on the space environment.