Spec maps have no effect on metals. Usually a single number value is plugged in into the specular node if you want to change something (although most times it's just left alone). The opposite of metal in UE4 is the 0 value. So you use that, and continue your roughness/normal/AO workflow as usual.
Its important to note that the "specular" input in UE4 isn't a specular input at all. Its more like a cavity map. Its really confusing, but don't even think of that as specular as its something completely different. People do still use specular intensity maps, but not in UE4, and generally not along with the metalness…