First of all, GTA is not an RPG. Second, Fable is only good because of Peter Molyneux, who also made Black and White, which is a fucking kick ass game.
Actually, GTA is an RPG in a loose way, like The Legend of Zelda. Here are several RPG traits GTA 3 has:
1) you play the
role of a character who doesn't speak and you choose to an extent his personality. ie. should I obey traffic laws or not? should I practice random violence or not?
2)you collect stuff. see pokemon and countless other RPGs.
3) the new GTA will have more character attributes you can build and shape, but even in GTA3 your endurance can increase/decrease based on how often you jog instead of driving the car.
4) there are multiple ways to accomplish objectives, again, role-playing is all about
making choices.
Third, there is no GTA 4, if you are refering to the latest GTA which comes out in October, GTA: San Andreas, you would be incorrect as well as this would be the fifth game in the Grand Theft Auto series. If there was a GTA 4, it would most likely be GTA: Vice City. Fourth, you lose.
actually, which ever game is considered the next-gen GTA will prolly be called GTA4. Both GTA1 and GTA1 had scenario packs ie 1960's London that didn't receive new names. While the scale of GTA3's sequels are much bigger than scenario packs, they still follow the same basic formula and have the same engine. At some point, at least based on their historical naming process, there will be a GTA4.