One thing about CD4 discrete quadraphonic records--they were half-speed mastered. In fact, that's how MoFi got into half-speed mastering. Half-speed was created to be able to cut the back channels which were modulated up to 40KHz. When quad tanked, MoFi acquired one or more half-speed machines and started mastering standard 2-channel with them.
IIRC the Shibata tip was developed for discrete quad replay. That doesn't mean a good elliptical won't work. It would be worth looking up some vintage quad cartridges to see what the typical stylus shapes and compliance specs were. Any cartridge today with a similar compliance and stylus should be fine.
