How would one go about making an FE4 negative growths patch, if such a thing is even possible

I am pretty new to FE hacking. I’ve messed around in the gba editor, but that’s pretty much it. I know vaguely how to make such a thing in the FE8 engine, but I was wondering if it was possible for FE4, and if so, which program I should/could use to make it (or if it’s already been done before).

I believe you’d have to alter the level up routine.