You don't have to read the Service Manual unless you're a service technician fixing the machine!
The User Manual is very "fact lite" but it will get you started, just about.
The Reference Manual is where all the detail is, but it is rather tediously translated to English from Japanese... and more importantly, it was NEVER updated to include all the little (and not so little) changes to the OS in all those "Update Addendum" PDFs.
So yes, you really do need to read all the Addendums!
At one point I made an annotated PDF of the 4000 Reference Manual, with little "post it" note comments on it for all the changes... it should still be in the Files section of the MPC4000 Yahoogroup.
Another great document is the Ak.Sys User Guide. Unlike all the rest, it was written by Steve 'Hollow Sun' Howell (who sadly died last week

)
It is not an exhaustive reference to everything the MPC4000 can do, but it explains many concepts very well in a much more readable way than the Reference manual.