Most non-server editions of Windows 32-bit (X86) are restricted by a 4Gb memory limitation.
When you install 4Gb or above into a machine with a 32-bit version of Windows the amount of usable Memory (RAM) available will vary between 3Gb & 3.5Gb.
The reason for this is because Windows 32-bit cannot address more than 4Gb of Memory (RAM). You will then loose a small percentage of memory for the use of the on-board graphics and/or video card, although the amount of memory these devices use can be adjusted in the BIOS (consult you computer handbook for details).
There are other versions of Windows that also suffer from memory limitation. Click here to read about memory limitations in Windows Starter Editions.