I think for something like RetroPi 4GB could be very useful when trying to run some of the more demanding emulators and games but you also need to think of it this way. This whole time everything Pi related has been with the previous memory limitations in mind, and people get lazy when they have more to work with. I don't need to optimize this I have 4GB ram to play with. Kind of like a cartridge game vs the first game on CD, or the first games on DVD so much more to work with so who cares about texture optimizations we have room to spare. I think eventually usages will pop up that will be 4GB minimum and run away just like everythign else.