so just to clear some things up…How do you light your scene and how is the scene built? (Do you need indoor outdoor transitions or not, do you use the movable directional light?)
Because the thing is this: AO in UE4 gets only applied to indirect lighting. So if you fake GI with point lights, its again direct light so the AO gets masked off in those areas = its not visible. UE3 did not have that functionality, so faking GI with point lights was easy.
What you need to do (and that is why I asked how your lighting setup is) is to fake it with image based lighting. The thing important to know here is: are you using the flag “force no precomputed lighting”? If yes, you can only use the ambient cubemap in the post volume and this one doesnt have lower hemisphere shadowing nor sky occlusion (which means interiors inside of buildings, caves etc. will receive the full indirect light amount from the cubemap)
If you dont use it, you can still use the skylight in a semi dynamic mode (place it but dont bake it) and you will get at least the lower hemisphere shadowing, which makes it look better that the AC from the post volume, but you will still not get nice sky occlusion.
If you only do outdoorscenes right now…it doenst matter. However, as soon as you have interiors, the lighting there gets pretty much destroyed by the IBL, becaus most of the time you will use something bluish, because of the sky and so on^^…and then your interiors will be filled with blue bounce light from the sky, which looks very ugly^^
So yeah…if you go fully dynamic, just use the Ambient Cubemap/skylight to brighten the shadows with fake bounce light from an image (just import one of the many free HDRi images from the web) and you will have full AO that you will definitely see^^
Ah and BTW…try out the LPVs. There is still alot of stuff missing right now (no sky lighting, no reflection support and a lot of other things) but its a good start to test out and it makes fully dynamic GI from the Directional Light (point and spot will be added later)
Hope that helps and cheers!