Shape keys in Blender (Morph Targets in UE4) are incredibly useful but there are still some issues with the Blender to UE4 pipeline to be aware of.
Selecting “Faces” for smoothing in the FBX export can result in UE4 crashing, selecting “Normals” works around this (but obviously may cause you other problems with smoothing) – apparently it was fixed in Blender a couple of months ago but doesn’t seem to have made it into the version I’m using (2.74) as I am getting UE4 crashing. So you may find this unnecessary.
“Apply Modifiers” can apparently cause issues with shape keys.
Scaling a group of vertices down to 0 so they overlap at exactly the same coordinates can cause the shape keys to not work in UE4, scaling to something very very small like 0.01 will make them practically invisible without breaking the morph targets in UE4.