This could be made possible by the use of @item2 (which would be much like the intervention of a NPC) or else you'd need a source edit or making a special item that compounds on slot 4 of armors and equipments. That source mod wouldn't be nice because you'd need to find a mean to make the server know what items are enchantable (meaning hardcode if it's made by someone unexperienced) and a lot more checks.
This also could make a big problem on card withdrawal NPCs.