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 monaural (audio & cue tracks, later 3-track for stereo) tape cartridge system invented by George Eash in 1954 and used by radio broadcasters for commercials and jingles in 1959. The Stereo-Pak was adapted from the Fidelipac cartridge design by Earl "Madman" Muntz in 1962, in partnership with Eash, as a way to play pre-recorded tapes in cars.

The tape is arranged in an infinite loop that 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.

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 automobile record players such as the Highway Hi-Fi and the Auto-Com flexidisc.

Entrepreneur Earl "Madman" Muntz of Los Angeles, California, saw a potential in Fidelipac broadcast carts for an automobile music tape system, and in 1962 introduced his "Stereo-Pak 4-Track Stereo Tape Cartridge System" and pre-recorded 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.

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