@MostHost_LA: I’m afraid that it won’t work to brighten only one spot, because it won’t be seamless and the skin texture now is very pale as it is now.
Furthermore the size and edges of the shadowed area depend on the angle of the rays from the light source in relation the spot (armpit are) affected.
Neither did the use of a subsurface skin profile nor using the tessellation multiplier (parameter value of 3, geometry got very fine) help in removing the shadow.
So this is no problem of the mesh itself.
@FrankieV: I’m in a situation, where it is a problem already as everything else seems to work just fine and it is really visible on my characters from “FPS” perspective.
As for smoothing groups I don’t have any issues with that in UE, because blender has the option to set the smoothing group for example to “edge” before exporting an fbx file,
so my mesh does contain smoothing group information during import into UE.
Even exporting without smoothing group information and thus regenerating them by ue did nothing.
By the way: Did you know that it is now possible to import daz characters (Genesis 8, Genesis 3) with all ~94 jcms into UE4 and even without any subdivisions it behaves and deforms in a clean way, so no need for further workarounds, adjustments , skin reweighting, resculpting or any other tricks.
But you need the dual quaternion version for it to look and behave like in daz. You can even import expressions etc.
The necessary tools are blender + diffeomorphic blender plugin. JCMs are bone driven already, which means that there is no need for adjusting your animations or morph target curves in ue4, because you can do everything in blender and just export it.
The workflow is really fast, so in my opinion DAZ / Genesis is really easy to use with UE 4, once you have learned what to do and follow a specific order during the workflow.
As for hair I wouldn’t want to use any daz hair in ue, because there are better solutions like taking hair from sims 3 (or some downloaded hair for sims 3),
which looks very good too and needs only ~10 k triangles maximum.
@all
Back to the “shadow”:
To reproduce the problem:
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Just load the standard genesis 8 or 3 character into ue4 with all jcms applied (because you won’t see the shadow without jcms)
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Create & import a pose with arms up
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Pose your character against the sun, so the light shines onto the armpit
Enough of guessing 
Follow these steps and only then you will really be able to see what I mean. You can even use the non dual quaternion version of UE for seeing the same shadow glitch.