The Yahoo! group has a spreadsheet file with a short listing of usable drives. It is over a year old. The drive model and manufacturer are included in the file. You might be able to cross reference the drive specs with other more popular drives. To the best of my knowledge AKAI has never published any definitive specs for compatible drives.
http://groups.yahoo.com/group/mpc4000/files/
Read the Version 1.20 Addendum of the User Guide for a description of feature enhancements offered by the drive. The Version 1.40 Addendum contains a few other operations that might interest you.
There are some individuals who think that AKAI was/is going to offer audio mastering for the CD-R/RW drives. This misinformation began circulating when Roland released the MV-8000 with audio mastering.
The AKAI MPC4000 brochure referred to “master archiving” as a feature. This is much different from “mastering” an audio file. AKAI delivered the MEMORY TO CD master archiving in OS Version 1.40.