They exist(ed), but it almost looks like they aren't made anymore. The guitarist in my old band had a Tascam 8-track cassette Portastudio, but Tascam's site doesn't have an 8-track cassette model listed any longer. http://www.tascam.com/PersonalCreativity.html
Tascam made a few models, the 688, 488 and 488 MkII which were all "portastudios" with mixers and the 238 which was a deck only. You can find them used and often quite cheap, like $250 or so.
They do have really thin tracks so the sound is actually not quite as good as a cassette 4-track.
