Not enough information of how you have things set up in UE4. As long as you can import back into UE4 with out errors then 3ds Max or Maya is not the problem.
This is one of those problems that has more than one possible solution and knowing how the player or character Blueprint is set up is probably where your problem is originating.
Assuming that both the body and hands share the same BP the quick fix would be to set the body to invisible while in game in it’s details panel.
P.S. if you just want to replace the body with the hands this should help.
The part on how the weapon was replaced is what you are looking for as you would import the hands as a new skeletal model using the same rig as the body and in the BP tell it to use your new model.