The Muntz Stereo-Pak, commonly known as the 4-track cartridge, is a magnetic tape sound recording cartridge technology.

The Stereo-Pak cartridge was inspired by the Fidelipac 2-track later 3-track tape cartridge system invented by George Eash in 1954 and used by radio broadcasters for commercials and jingles  beginning in 1959. The Stereo-Pak was adapted from the basic Fidelipac cartridge design by Earl "Madman" Muntz in 1962 with Muntz partnering with Eash, as a way to play prerecorded tapes in cars. 

The tape is arranged in an infinite loop which traverses a central hub and crosses a tape head, usually under a pressure pad to assure proper tape contact. The tape is pulled by tension, and spooling is aided by a lubricant, usually graphite.

History
The endless loop tape cartridge was designed in 1952 by Bernard Cousino of Toledo, Ohio. 

Previously, music in the car had been restricted mostly to radios. Records, due to their methods of operation and size, were not practical for use in a car, although several companies tried to market an automobile record player including the Highway Hi-Fi and the Auto-Com flexidisc 

Entrepreneur Earl "Madman" Muntz of Los Angeles, California, saw a potential in these broadcast carts for an automobile music tape system, and in 1962 introduced his "Stereo-Pak 4-Track Stereo Tape Cartridge System" and prerecorded tapes, initially in California and Florida. He licensed popular music albums from the major record companies and duplicated them on these 4-track cartridges, or CARtridges, as they were first advertised.

Music came in four cartridge sizes. 
1 AA (single) size was an inch wide by two inches long and carried the same amount of time per track (6 minutes) as a 45 RPM EP per side.
2 A size, 4 inches wide by 5 inches long the most common was the same size as the vast majority of NAB (Fidelipac) carts and able to carry a 3-inch reel.
3 B size, six inches wide by 7 inches long was used very infrequently for 2-LP sets and other extended programs and able to carry a 5-inch reel.
4 C size, the largest, able to carry a full 1800-foot 7-inch reel of one-mil tape, used also very infrequently for extremely extended 4-LP sets.

Muntz also developed and marketed a variety of mobile and stationary players and recorders for his 4-track tapes.

The B-and C-size carts would have their stereo sound split to mono and be used for background music systems all the way up to the early `90s when digital took over.
In the last part of that period, a last-ditch effort to reduce cost came in the form of reducing the tape speed first to 1-7/8 IPS and then to 15/16 IPS while reducing the cartridge size first back to the standard widely available A-size and then to a hybrid size between the AA (single) size and the standard A-size. First Chromium high-bias tape was used to offset the loss of fidelity from the lower speed, and then when that proved too expensive, cobalt-based tape was substituted.

After riding in Muntz's car, listening to his 4-track cartridge system, electronics and aerospace entrepreneur Bill Lear had the a staffer at Lear Jet Corporation create a modified derivative, resulting in the more convenient and long-playing 8-track cartridge system, which quickly supplanted and surpassed the 4-track in the market -- until being surpassed, itself, by the cassette tape system.

References

Audiovisual introductions in 1962
Audio storage
History of sound recording
Tape recording
Discontinued media formats
American inventions
1962 in music
1962 in technology
Products introduced in 1962