The cone was composed of a thin fiberglass shell coated with a gold wash to make it electrically conducting and painted with white stripes to maintain the temperature between 10 and 50 °C.

The Laboratory's Microlock system, used for communicating with earlier Explorer satellites, did not have sufficient range to perform this mission.

On top of the Jupiter propulsion section was a guidance and control compartment that supported a rotating tub containing the rocket stages 2, 3 and 4.

This time, the booster performed almost perfectly so that Pioneer 4 achieved its primary objective (an Earth-Moon trajectory), returned radiation data and provided a valuable tracking exercise.

A slightly longer than nominal second stage burn, however, was enough to induce small trajectory and velocity errors, so that the probe passed within 58,983 km of the Moon's surface (7.2° E, 5.7° S) on 4 March 1959 at 22:25 GMT (5:25 p.m. EST) at a speed of 7230 km/h.

The probe continued transmitting radiation data for 82.5 hours, to a distance of 658,000 kilometres (409,000 mi),[5] and reached perihelion on 18 March 1959 at 01:00 GMT.

The communication system had worked well, and it was estimated that signals could have been received out to 1,000,000 kilometres (620,000 mi) had there been enough battery power.