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The cassettes with 4 "sides" are actually just regular cassettes recorded to work with specialized players. On a regular commercial cassette, there are two tracks for each side, one for the left speaker and one for the right. This is nice for music, but not really necessary for an audio book, so each track on the tape is a different part of the recording. After the first side of the tape, you flip it over as you would normally, but after the second side, you flip it over to the first side and then flip a toggle switch on the player to get to the third "side" of the cassette. They are also usually recorded to play back at 15/16 ips (a regular commercial cassette is 1 7/8 ips.) The National Library Service in the U.S. uses tapes that would hold 90 minutes of audio if they were used in the normal manner, but hold 6 hours in the specialized format.
Unfortunately, because cassettes are on their way out, it's becoming prohibitively expensive to get blank media and replacement parts for cassette players, so the U.S. is switching to digital audio books that will be distributed (with DRM) on cassette-sized flash cartridges and downloadable from the NLS-BARD web site. I have been fortunate to be able to participate in the beta testing of the digital books, and they are wonderful (except for the DRM that is required under the copyright law provisions that allow the program to exist).
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