Wonder why can't you use full of your system's RAM ..!!

• Looks like you're using a 32 bit operating system windows . Because a 32-bit operating system has only enough addresses to handle 4GB of memory.One you past that , it doesn't know how to handle it.
• To check whether your system run with 32-bit or more,  try this                                                         Computer (right click) -- Properties  (or) Press Winorb + Ctrl/break button together.

• If you have 32-bit operating system, you still can't use the full 4 GB because the OS needs some of those addresses to interface with the hardware.Usually it takes half or more than a Gigabyte, exact amount may vary.
• So, to take full advantage of 4GB, you need a 64 bit operating system. Of course the latest versions of windows come in both 32 & 64 bit versions.
• But beware, there's a chance that you might not have the right hardware to install 64 bit version. To make sure, download the Gibson Research's free program, https://www.grc.com/securable.htm.Securable . If the program displays a big 64, you can then upgrade to 64 bit version of windows.

• And just to be safe, you should make full backup before doing the install.