It's basically just because of the way Windows 7 (and Vista for that matter) distributes RAM..

If you do actually have 4GB's installed then no worries, you have 4GB's available to use, it's just divided up as Windows sees fit..

It's not really a "problem" as such..
If you feel like reading there are a few threads on this forum that describe how 4GB RAM or more works in Windows..

I'll try to find them and post the links..