in reality that 4gb will never get used up unless one goes nuts with the skyrim mods.
and also, if there is a game that needs 4gb of video memory then chances are the gpu will be too slow/old to handle it, let alone use up the memory. (i don't see this happening for a while as the higher the requirements of a game, the fewer potential customers there are).
you mentioned that the 4gb variant is 1.5k more. if you plan on changing the gpu within the next 3 years then the 4gb will net you better resale value.
other than that, the 4gb's advantage is very situational.