I have something in the back of my C64 that runs it at 4x speed...
Sounds like a Schnedler Turbo Master CPU 4.09 MHz accelerator cartridge.
...and has high speed DOS as well.
Yes, the Turbo Master has a type of DOS that works at 4 MHz. and 1 MHz. (whichever you select on the DIP switch).
Mine is somewhere in a box,
Robert Bernardo
Fresno Commodore User Group
http://www.dickestel.com/fcug.htm