I am not certain as to how the UE4 does AO, but I recall it being a multiply into the diffuse and specular ( it might just be specular). Not just an occlusion of ambient light which is certainly how Marmoset's AO works.
Just bumping this thread again as a PSA: I did some tests in UE4, and found the difference between using AO is night and day. Here is AO when being lit directly by the sun and skylight, Here is AO when the sun is facing away from the object (so only the skylight is providing light) As you can see, you need AO to provide…