In most cases current PC games do not use more than 1GB of VRAM (all settings maxed) when running at 1920x1080. The only real need for 2GB or more of VRAM is if you are running games at 2560x1440 or higher. Larger amounts of RAM are needed to cache incredibly high res textures. However, at those resolutions (2560x1440+), even dual GPU's (with RAM sufficient to handle the textures) are unable to achieve fps that i find playable (60+).
So the design questions/considerations really become:
Q: what resolution
A: next gen consoles are being designed around 1920x1080 (1080p HD TV's)
Q: what fps are we trying to achieve
A: 60 FPS (assumed)
Q: how much ram is needed to accomplish questions 1 & 2
A: more often than not, 1GB is sufficient to run current high fidelity PC games
Q: how much ram is needed to absolutely run games at 1920x1080
A: 2 GB is a safe number
Q: how can we future proof our console
A: lets thrown in 4GB of DDR5.
I've got a 2GB of DDR5 on my N670 and only Max Payne 3, Skyrim, and Crysis 2 cache more than 1GB of VRAM. And thats with full tessellation, texture res, and so on.
I really don't see a need for the PS4 to have more than 4GB of ram unless it turns out a significant chunk of it is taken by the OS. At launch, if the system allows 75% (3 GB) usage by games and 25% (1 GB) usage by the OS, that will equate to a very powerful console. As the years go by, Sony may be able to streamline the OS and reduce its RAM requirement.