is there a...

[Sylvie]

Is there a tool for hacking the FF4 rom like there is for FF1?

[Starboard Regulator FlareNUKE]

Not that I am aware of... though there is this.

http://blitzkrieg.finalconquest.net/ff2usme.html

I've yet to use it, but it seems like this is what you want.
Reminds me of FFHackster a bit...