"What was the score with putting the Mach 5 internals into the Manitou 4 fork?"
I have looked into that. I tried... but unfortunately, whilst the tube diameters are the same, they did change the bushing design from Manitou 4 to Mach 5 dramatically enough that the Mach 5 uppers cannot be transplanted to the Manitou 4 lowers and crown. This could have been a cool and cheap way to upgrade your Manitou 4 or 3 to an oil damper and steel (or Ti) springs (Mach5 forks aren't very popular and they can be purchased cheaply).
That being said, the EFC internals are compatible with the Manitou 3 and 4 lowers (sort of... you do have to drill a bigger hole at the bottom of the right leg where the oil damper is, but that is a very easy fix). I do own a Manitou 3 fork that has the EFC internals transplanted to it. I also got rid of the elastomers and used a Titanium spring kit from a SX-Ti fork... and voila: Manitou 3 looks but a more modern performance.