Stereo tape recorders record two tracks at a time on a cassette.A four-track recorder allows you to record one track at a time while listening to the other three. This allows you to record a guitar part, then add a vocal part, then add a harmony, and so on. The Beatles recorded several albums using only four tracks. Four-track cassette recorders were the first affordable multi-track recorders for home musicians. Multitrack recording software, like Garage Band and Pro Tools, allow you to record many tracks, but the principle is the same. FourTrack (the iPhone app) gives you the simplicity of the old-school cassette four-track on an iPhone. What else we can have, iPhone apps developers are doing lots of thing for us.