In the 32-bit version of Windows 7 from 4 GB usually only 3.25 -3.5 GB of RAM are usable:
Here a patch which can let you use most of the memory, depending of your windows version
I've test, I've 4 Go installed( was 3,25 usable) it's now 3,75 usable, always cool
read the instructions carefully, but it's quite easy
it's create a new extra boot menu entry in the Windows 7 boot menu, so you can test easily
Check Here