I'm not sure what you mean to ask. Do you mean that you have a total of 4 GB of RAM, but Windows can't see more than about half of it? Or that it can see it all, but reports that only half is "free" for use?
In the first scenario, you're using 32-bit edition w/o PAE enabled.
In the second...Windows 7 manages memory in a different way from pre-Vista versions, and what seems like a large system footprint isn't really a problem.