CD-4 is a discrete 4 channel quadraphonic system and uses a separate FM carrier for the rear channel modulation, playing a CD-4 recording with a conventional cartridge will wipe the carrier off the disk in short order. The other two SQ and QS are matrix based quad and used the instantaneous phase relationships between the two channels to determine what channel was being played at any given moment. The better decoders used a combination of amplitude and phase information to steer the signal to the appropriate channel. Separation was not very good.
I find it extremely difficult to believe that Acoustical Manufacturing Corp (Aka Quad) in that time frame had anything to do at all with the design and manufacture of phono cartridges sold on japanese turntables. Panasonic (Matushita) for example made a whole bunch of strain gauge based CD-4 cartridges in Japan. It would be like bringing coals to Newcastle imho.
In the 1970s in the U.S. anyway, "QUAD" was understood to refer primarily to quadraphonic sound, most non-audiophiles in here had never heard of QUAD (the company), although it is very well known and respected in audiophile circles. The cartridge in question is a CD-4 Quadraphonic cartridge, which I doubt was made by Toshiba, more likely Panasonic, AT or Denon, or perhaps one of the many more obscure makers that have existed and continue to exist in Japan to this day.
To the OP, have you looked at what Jico has to offer?
http://www.jico-stylus.com/
(Note I was not able to find a match) You may want to investigate further and look to Japanese sellers for possible sources.
https://quadraphonicaudio.wordpress.com/2009/07/12/cd-4/
http://www.4channelsound.com/cd-4.htm
This doesn't appear to be a P-Mount cartridge and can be replaced with any standard cartridge.
