There isn't anything that will be as accessible as FF4kster (which is what I assume you've been using for the SNES version).
I haven't had any experience hacking the GBA version, though I think I've heard it's structured similarly to the SNES version in many fundamental ways. If that's true, well, it doesn't bode very well for what you're trying to do unless you've got some experience ROM hacking with a debugger and a hex editor.
A quick search of this site reveals only this as far as documentation of FF4A's text structure:http://datacrystal.romhacking.net/wiki/Final_Fantasy_IV_Advance:ROM_map
I don't want to discourage you. If all you want to do is edit the game's text, it's definitely something that can be done. If you don't have a lot of experience, this is the kind of project that is a good entry point. But you should first do some research about ROM text editing in general - get familiar with the concept of "pointers," as manipulating them will be as essential as is finding and editing the text itself.
Other folks, who actually have GBA hacking experience, will be more help than I have been, so I'm passing the baton now.