before the 8 track, was a 4 track

2 sides... when you flip the cartridge over it would have the magnetic tape reading heard on the other half of the tape, and the half tape was divided into left and right so you heard the music in stereo

8 tracks were an improvement brought on by new tech ways of making smaller read heads that were 1/2 the size

and everything I wrote above is speculation based on obsevation. Thanks Steve!

"The four tracks were divided into two "programs", typically corresponding to the two sides of an LP record, with each program comprising two tracks read simultaneously for stereo (two channel) sound playback. He licensed popular music albums from the major record companies and duplicated them on these four-track cartridges, or "CARtridges", as they were first advertised."