You have to cut apart the pad you are modding and the pad with the donor parts,
Epoxy case parts together, and fill gaps with bondo
primer, paint and clear coat your game pad unless you like your game pads looking like they been though a auto shop.
and this does not even address the electrical modifications inside the pad
Alternative case modification methods would include cutting material on a CNC machine and 3D Printing
Yep. As far as I know, the only person who’s done it is @Gummo, although it was with a Fighting Commander 3 Pro, not 4.
Basically you need to trim down both a Mad Catz 360 Fightstick PCB and an ImpV2, and cram it all in there. If I recall, it’s a straightforward mod, but not trivial, due to physical (rather than technical/electrical) limitations.