Just as a floater:
Would it be possible to get any old 8 track player, put a q8 head in and somehow alter the cartridge with some tape over a slot and have a home brew 4 ch player?
It would all look a bit heath-robinson but for the odd archiveing project it should be ok.
You would need An old 8 track player which seem readily available on ebay.
Perhaps another player for some more replay electroncs.
or
Find a circuit e.g.
http://www.circuithut.com/index.php?/circuit/content/view/full/138 or
http://www.eed.usv.ro/misc/mirrors/cc/circuit.htm/0099.htm
a Q8 head (where would you find one ??, I've recall a site called headcase or something but can't find it again).
Perhaps a 12v power supply (assume USA cars are 12v neg earth?)
and some power for the circuits.
And some time.
I'd like to add I have never owned an 8 track player stereo or otherwise and only have a basic understanding on the physical difference between the 2ch and the 4ch cartridges.
As an alternative on heads.
Does anyone know what the track spacing is on a Q8 head? and how close would it be to a 4 track cassette head (e.g. like a porta studio).
I belive cassette tape is 1/8 inch wide, 8-track tape is 2/8 wide (i.e 1/4 inch) so if the 8-track tapes are not interleaved i.e. programme 1 is tracks 1,2,3,4 and programme 2 is 5,6,7,8 it might be possible to use a cassette head instead.