The cartridge could just be messed up. Opening it wont help, there's nothing inside (I dont know what your uncle meant by batteries, I've smashed opened cartriges from lots of systems and the catridges are really just window dressing. That long chip you see is really all that's in there), and unless you know what you're doing, opening up your SNES could further damage it.

It may be better to just play FFIV on PS1, it's more of a challenge and the translation is better anyway.