This actually has to be changed through the 3D model software(blender , maya etc). What you need to do is called UV unwrapping . After you do this properly and the way you want, then more material slots are going to appear in UE4.
I imported a fbx car file but it have only 2 slots to put material and I cant paint this because i cant paint the wheels with the same color as the doors. any help?
thanks____