Quick addition, as I just did another power usage test:
4GB Unit sitting in U-Boot: 0.38A (1.89W) 4GB Unit running mtest in U-Boot: 0.56A (2.8W)
2GB Unit sitting in U-Boot: 0.37A (1.85W) 2GB Unit running mtest in U-Boot: 0.58A (2.85W)
So it seems in full usage, the 2GB RAM uses a bit more power than the 4GB RAM.