The first thing is what are you going to be doing with it?
If it's just general internet use, emails, writing the odd letter ect then 2GB will be enough, if you are going to be doing things like video editing, or editing large photos then 4GB would be better.
Also I'd be asking what configuration the RAM comes in if you do get it with 2GB, the computer will have 2 RAM slots, if, when you buy it with 2GB, it comes with 2 x 1GB chips then you will have to remove one or both of them if you decide to upgrade in the future, if it comes with one single 2GB chip then you can simply add to it at a later date.