For completeness sake, passing along information on what the spec input in UE4 does: "It's used as a masking function on specularity. So it is really a cavity map. Anything above 50% grey gets extra highlights everything below gets AO. "
UE4 does not use a specular map, it uses metalness which works by storing specular values in the Albedo. The metalness map is a mask that controls a behind-the-scenes blending between a constant specular value and the value in the Albedo (pue black uses the default specular & pure white uses the albedo's value). Then where…