Where did you get the number 4 gigabytes from then?
If you're referring to having 4 separate 1 gigabyte sticks of ram, then I must remind you they all have slightly less than advertised due to how math, gigahurtz, and ram works. So if you've got 4 sticks of ram with 1 gig each, it'd probably somehow come out to 3.5 or so in my experience.