GDDR5 is known for its bandwidth, not capacity--which is why the motivation for it was sufficient at 4 or 8 GB.
My question now is what Sony was planning on doing prior to there being 8 GB to carve from.
Yeah at 4 gb you are talking a very different machine it would seem. Stuff like sharing and game resume/switching and the like might not have been in the cards for that machine or they would have had to have had more discrete modules for different things.