It may also be worth noting that the Specular input does NOT respect colour information. I’m not sure what channel of the texture input it actually uses (I’m assuming red, but green has better DXT compression), but colour information into the specular input is NOT used. I’m not experienced with Marmoset at all, but I guess it uses a much more complex PBR system being a rendering program. UE4 is optimized as best as possible for quick framerates, and uses a much simpler system.
A detailed normal map might help the situation here, it looks like right now the normal map in UE4 isn’t breaking up the specular highlight as much as marmoset is.