Mariner 4 (Mariner C-3, together with Mariner 3 known as Mariner-Mars 1964) was the fourth in a series of spacecraft intended for planetary exploration in a flyby mode. It was designed to conduct closeup scientific observations of Mars and to transmit these observations to Earth. Launched on November 28, 1964, Mariner 4 performed the first successful flyby of the planet Mars, returning the first close-up pictures of the Martian surface. It captured the first images of another planet ever returned from deep space; their depiction of a cratered, dead planet largely changed the scientific community's view of life on Mars. Other mission objectives were to perform field and particle measurements in interplanetary space in the vicinity of Mars and to provide experience in and knowledge of the engineering capabilities for interplanetary flights of long duration. Initially expected to remain in space for eight months, Mariner 4's mission lasted about three years in solar orbit. On December 21, 1967, communications with Mariner 4 were terminated.

The Mariner 4 spacecraft consisted of an octagonal magnesium frame, 127 cm (50 in) across a diagonal and 45.7 cm (18.0 in) high. Four solar panels were attached to the top of the frame with an end-to-end span of 6.88 meters (22.6 ft), including solar pressure vanes which extended from the ends. A 104.1 cm × 66.0 cm (41.0 in × 26.0 in) elliptical high-gain parabolic antenna was mounted at the top of the frame as well. An omnidirectional low-gain antenna was mounted on a 223.5 cm (7 ft 4.0 in) tall mast next to the high-gain antenna. The overall height of the spacecraft was 2.89 metres (9.5 ft). The octagonal frame housed the electronic equipment, cabling, midcourse propulsion system, and attitude control gas supplies and regulators.

The scientific instruments included:

The electric power for the instruments and the radio transmitter of Mariner 4 was supplied by 28,224 solar cells contained in the four 176 cm × 90 cm (69 in × 35 in) solar panels, which could provide 310 watts at the distance of Mars. A rechargeable 1200 W·h silver-zinc battery was also used for maneuvers and backup. Monopropellant hydrazine was used for propulsion, via a four-jet vane vector control motor, with 222-newton (50 lbf) thrust, installed on one of the sides of the octagonal structure. The space probe's attitude control was provided by 12 cold nitrogen gas jets mounted on the ends of the solar panels and three gyros. Solar pressure vanes, each with an area of 0.65 m² (7.0 ft²), were attached to the tips of the solar panels. Positional information was provided by four Sun sensors, and a sensor for either the Earth, Mars, or the star Canopus, depending on the time in its spaceflight. Mariner 4 was the first space probe that needed a star for a navigational reference object, since earlier missions, which remained near either the Earth, the Moon, or the planet Venus, had sighted onto either the bright face of the home planet or the brightly lit target. During this flight, both the Earth and Mars would be too dim to lock onto. Another bright source at a wide angle away from the Sun was needed and Canopus filled this requirement. Subsequently, Canopus was used as a reference point in many following missions.

The telecommunications equipment on Mariner 4 consisted of dual S-band transmitters (with either a seven-watt triode cavity amplifier or a ten watt traveling-wave tube amplifier) and a single radio receiver which together could send and receive data via the low- and high-gain antennas at 8⅓ or 33⅓ bits per second. Data could also be stored onto a magnetic tape recorder with a capacity of 5.24 million bits for later transmission. All electronic operations were controlled by a command subsystem which could process any of 29 direct command words or three quantitative word commands for mid-course maneuvers. The central computer and sequencer operated stored time-sequence commands using a 38.4 kHz synchronization frequency as a time reference. Temperature control was achieved through the use of adjustable louvers mounted on six of the electronics assemblies, plus multilayer insulating blankets, polished aluminum shields, and surface treatments. Other measurements that could be made included:

Mariner 4 was also supposed to carry an ultraviolet photometer on the left side of the aft TV Camera scan platform. Late in testing, it was discovered that the inclusion of the UV photometer produced electrical problems that would have jeopardized the TV Camera. As a result, it was removed and replaced with a thermal/inertial mass simulator that was designed to emulate the UV photometer's geometry, mass, and other characteristics so that any unintentional problems caused by the removal of the UV photometer would be negated. This spare UV photometer was eventually flown on Mariner 5 in 1967.

Mariner 3 had been a total loss due to failure of the payload shroud to jettison. JPL engineers suggested there had been a malfunction during separation of the metal fairing exterior from the Fiberglas inner lining. Pressure differential between the inner and outer part of the shroud could have caused the spring-loaded separation mechanism to become tangled and thus fail to detach properly.

Testing at JPL confirmed this failure mode and an effort was made to develop a new, all-metal fairing. The downside of this was that the new fairing would be significantly heavier and reduce the Atlas-Agena's lift capacity. Convair and Lockheed-Martin had to make several performance enhancements to the booster to wring more power out of it. Despite fears that the work could not be completed before the 1964 Mars window closed, the new shroud was ready by November.