4.7 is only the max for residential use. Industrial dvd-burners use the multi-layer feature, which gets at least 7GB on one side. Plus, they're coming out with a new algorithm that gets 25GB per side. That's out sometime not too distant futuristic. They're also coming out with something like a HDD that goes down to the size of atoms(no shi7, seriously!) It'll have a capacity of something like 25/250GB per inch2 I forgot which, but I'm thinking it's more toward the upper number.
So, my point is, I'm hoping that PS3 will use this new algorithm, or the new format that's a card-form, not disk-form. But GT4 is for PS2, so they can use up to 14GB per disk if they wanted to...