Theorycrafting - Turn FF6 into FF4?

Adding a 5th battle party member would be a real ASM nightmare. You'd also need to consider adding him as a regular party member, another nightmare lol. If you stick to 4 party member, twincast would be one of the hardest battle hack to implement. "FF4 with FF6" is definitely doable if your goal is to make things more or less close, how close to FF4 it will be depends on the amount of hours you're willing to put into it.

As for hacking FF4, you might want to look at what FF6Tools allow to edit.