Possible problem, as far as I know the higher capacity cells, 4ah, are volumetrically more dense but not gravimetrically, so that larger pack will add some weight, and hurt efficiency.
Maybe I'm misunderstanding, but that doesn't quite make sense to me.
You're saying the 4 ah cell is smaller and heavier, but the cells are still 18650 format so the size doesn't change. I suspect there is a gravametric improvement. In the London video, Elon speaks to batteries becoming more energy dense gravametrically, saying that he sees a clear path, over the next 5 years to around 280 Wh/kg. It's common to here of gravametric improvements, but quite rare for anyone to mention watt-hours per liter.