I assume your baking on max quality? Look like it’s from the light bake rays where you just aren’t getting enough rays in this area. You can increase the quality which should increase the number of rays used. You can probably also adjust some of the ray number settings in the config files (as was the case in UE3)
With light calculations like this when your main light is several bounces away you can get this kind of degradation easily. One less cpu intensive solution is to place some of your own soft lights to simulate some bounce light yourself just having these extra subtle lights can help.
One thing to keep in mind is that once you have textures etc. (assuming you will) on these object the splotches will be less noticeable as well.