During normal use (screen active, cpu somewhat active), I don't expect there to be a big difference in power consumption between 2GB RAM and 4GB.
During standby (lid closed, cpu idle), I expect a quite noticeable difference in power consumption, since RAM is pretty much the only thing that stays powered in that case (except maybe wifi/3G, which could also be power drains), and since it's not going to be low-power RAM and RAM power consumption is pretty much proportional to its size. Suppose 50% of the power consumption during standby is spent on RAM refreshes, then having twice as much RAM would mean adding 50% to the standby power consumption, which means that if it would be 9 days with 2GB RAM, it would be reduced to 6 days with 4GB RAM.
Unless of course there's a way to power down part of the RAM, and let the kernel do this automatically using some kind of heuristics (e.g. if it's idle for some time, it could sacrifice some caches and reorganize the memory layout in order to be able to temporarily power down half of the RAM). I have no idea if such a thing is possible and/or implemented in Linux.