I’m planning on using the UE4 plugin “Allright Rig” to set up a rigged character in UE4.
I don’t know if you are already familiar, but when you make blendshapes for the mouth corners, some of the shapes you create are a “Mouth Wide” blendshape which pulls the mouth out, and you also create a “Mouth Up” blendshape which pulls the mouth up. When you activate them together it forms a smile, but usually the result looks messy, so you can sculpt a “fix” which also activates a third blendshape in conjunction with the first two blendshapes. It it is activated based on the multiplied values of the first two blendshapes and smoothly transitions in and out according to the values of the first two blendshapes ranging between 0-1.
It’s really simple to set up in Maya with the click of a button, but I don’t know how hard it would be to set up in UE4.