You can build a bone for each part of a body that needs to flap around, then in ue4 assign physics constraints to that bone in the physics asset of the skeleton. But it’s something that takes a whole lot of time spent tweaking to get looking right (I spent 6 hours going back/forth between play mode and the physics asset window). Also need some care when weighting your bones. Blender made it a bit easier on me because you can add bones to the hierarchy without the weighting getting too mad (something 3ds max gives me trouble with).
I don’t know if this is the best method, but it seems fairly lightweight performance wise.
For reference. if I have some upper arm flab so shake around, I’d build a bone jutting out of the center of the flab mass at the intersection with the bone, then angle the end of the bone to just run out the end of the mass. In the physics asset, you would make sure there is a collision generated for that bone. Make sure to position it so there’s no intersection with the arm collision (you can shrink and position either of them as needed). Then select the flab collision’s physics constraint (it should be pregenerated in the skeleton tree on left side of the window). Set linear limits to off, set the angular limits to limited, with ~20 degrees of freedom (you might want to keep one or two axis at locked, but for testing purposes, keep all 3 at limited). Drop the skeletal mesh asset into the level, play simulate, pick up the mesh and shake it around to make sure the physics is responding. You may want to turn on the SLERP angle target motor, and set the strength to something like 50000 (depending on collision mass and intended rigidness). I’ve had no good experiences using soft limits.