The reason why "2GB of GDDR5 in a unified architecture" with the provision that "we may have 4GB if densities increase in time" was mentioned in target specs is because it's using a faster type of memory that only comes in lower densities, unlike Microsoft's console.
There have been rumours that Sony's internal development teams are pushing Sony to bite the bullet, density increase or not, but those are the rumours. The console is launching late 2013, so whatever is available to them reasonably will be where they go if I were to guess. lostinblue is correct in that adding a lot of complexity to your motherboards impacts future cost reduction greatly and after this generation Sony (and Microsoft, and Nintendo) are very sensitive to that. Die shrinks will be much less of a factor than it was in previous generations as well, so initial price matters.