That is not how it works.
For a 256bits you need multiple of 8 modules of memory.
For PS4 you have only 512MB (4Gb) modules in the market so 512MB x 8 = 4GB but Sony bet the 1GB (8Gb) modules will be released before the PS4 launch so they could get 1GB x 8 = 8GB (that is what they call bold move... Sony bet in a product that didn't existed on the market).
Actually we already have 2GB (16Gb) GDDR6 modules in the market:
K4ZAF325BM-HC14 e
K4ZAF325BM-HC16.
So 2GB x 8 = 16GB.
GDDR6 memory for 16GB RAM with 256bits bus already exists... it is even in mass production (
K4ZAF325BM-HC14 model).
Go to Product Selector here:
https://www.samsung.com/semiconductor/dram/gddr6/ (find Mass Production).