Hm... that's odd, I'm not having this issue.. You're sure you're running all of the ModMash mods? And not using any other assembling machine mods?
One last thing I could test (which I thought I did before) is have ModMash as an optional dependency in both mini and micro machines. This should force ModMash to load before either of my mods, in case there was some weird load order thing happening.
EDIT: Actually, I see that ModMash Splinter Assembling Machines seems to load its machine during data-final-fixes, which is the same step as my mod. So if my mod loads first, it won't see the ModMash assembling machines, and fail to copy anything past tier 3. But it will still reference the tier 4 machine as its "next_upgrade", which causes the error. By forcing my mods to load first, it should prevent the errors. (v2.0.3)