PBR means there is no diffuse, its replaced by Albedo. (so use the albedo one)
Specular is not (really) used in ue4, you need to use roughness maps.
If there is no roughness map the pack is not ue4 ready, but you could X-1 (one minus) the specular and get something semi-useful for roughness.
Metallic maps should be black/white, not grayscale, if they are its probably not a good map.
Heightmap is for displacement/tessellation mapping.
also, all this is google-able, easily searchable on the answerhub, and quite well documented on Unreal Engine 4 Documentation | Unreal Engine Documentation 