Explorer 4 was an American satellite launched on 26 July 1958. It was instrumented by Dr. James van Allen's group. 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 Explorer program is a NASA exploration program that provides flight opportunities for physics, geophysics, heliophysics, and astrophysics investigations from space. Launched in 1958, Explorer 1 was the first spacecraft of the United States to achieve orbit. Over 90 space missions have been launched since. Starting with Explorer 6, it has been operated by NASA, with regular collaboration with a variety of other institutions, including many international partners.

Launchers for the Explorer program have included Juno I, Juno II, various Thor, Scout, Delta and Pegasus launch vehicles, and Falcon 9.

Currently, the program has three classes: Medium-Class Explorers (MIDEX), Small Explorers (SMEX), and University-Class Explorers (UNEX), with select Missions of Opportunity operated with other agencies.