I've just finished re-calibrating my MP4 from top to bottom. Took me 2 hours. And in the course of doing so, I realized the importance of certain points. First, there are some mistakes in the service manual. Not really mistakes, but more misunderstandings. The whole manual, and the calibration section, is based on the assumption that an old version of the MP4 is to be serviced.

However my MP4, as well as a LOT of others, is a newer version. So there are a few extra steps to take while reading the service manual.
But how can you tell if you have an older or newer version
Simple, when opening your MP4, if you see a small circuit board on the left marked KLM327 then you have an old version. On the newer version, that board is missing entirely.

So for the new version on the KLM355 board, first there is no more VR11 filter keyboard tracking adjustment. That VR11 is now part of the final VCA adjustments instead.
Second, when calibrating the offset of each VCO, the wire color reference for connecting the DVM is completely different. But a correspondence chart is provided at the end of the manual.
But here they are for quicker access:

OLD NEW
VCO1 Red Pink
VCO2 Yellow White
VCO3 Blue Purple
VCO4 Gray Green

Also, the trim pots correspondence for the offset adjust is not indicated. So here it is:

VCO1 -> VR2
VCO2 -> VR1
VCO3 -> VR4
VCO4 -> VR3

Another extremely critical point, is that you have to FIRST calibrate the voltages on the PSU. Then, calibrate the keyboard CV generation by adjusting the Key Assigner KLM356 to very precise values (a high precision DVM is essential here).
Don't start the VCO calibration unless you have confirmed that the assigner is generating EXACTLY 1 volt per octave, as described in the calibrating procedure. Otherwise you'll have a very hard time getting things right.

Personally, I prefer to use a digital frequency meter instead of a tuner for the VCO calibration, and a keyboard frequency reference chart. Numbers don't lie. Also, it makes it easier to compare octaves.

The end result is not absolute perfection, after all this is not a digital machine. But I've managed to get the tuning on each VCO to within +/- .1 hz over the 6 octave range, which is more than acceptable. But it takes methodical work and patience, going back on previous trim pots several times to confirm their value didn't change.

So a well spent 2 hours, with an end result beyond my expectations.
Before I calibrated it, my MP4 showed a +/- 5.5 hz difference on all VCOs across the whole 6 octave range, and the up-normal-down transpose switch showed a +/- 6.2 hz difference ! It didn't sound good.

Hope this can be of some help for those courageous enough (and well equipped enough, a 4 digit DVM is essential) to undergo this precise procedure. Otherwise you'd better leave this kind of job to a professional, or end-up with a badly out-of-tune MP4. You've been warned.

Alain.