I apologize if this ends up being a bit outside of the realm of Metallurgy 4 Reforged, but recently while giving the Material Changer mod a spin, I've found that it's unable to edit tools or armor from the Metallurgy 4 Reforged mod, usually bringing up errors about how vanilla Minecraft air is not a valid tool/armor material for editing purposes. This might possibly be Material Changer's fault, but that mod usually has little trouble editing modded tools and armor, so there's a distinct possibility that something is amiss with the way Metallurgy 4 handles its materials.

An example of a typical error log is linked below, illustrating what happens when attempting to use Material Changer to edit Metallurgy 4 Reforged's copper armor:
gist

As per carbotaniuman's request, a screenshot of the error log as it appears in-game has also been provided:

These errors occur using the following versions of Minecraft and related mods:

• Metallurgy 4: Reforged: 0.2.0
• Forge: 14.23.5.2768
• Minecraft: 1.12.2
• Material Changer: 1.0.3

It may be worth looking into how Metallurgy 4 is handling its materials, as problems there would impact not only compatibility with Material Changer, but a number of other tweaker-type mods as well.

@carbotaniuman

I can reproduce this bug. I just pushed a minor change (#165) to the Metal system but I'm not sure if that caused the problem. The big problem is that the mod names chestplates "_chest" and leggings "_legs".