right click the windows start icon on the bottom left
run
winver
and paste the screenshot here so that some of the experts here can check your
version

sorry but mind telling us again how do you show that only 4gb is being used?

where does it show it / how did you suspect this and from which screen?

good but how about this way

control-alt-delete
task manager
at the bottom click Open Resource Manager
then click on the memory tab
and look for this screen and the info

it shows 4096 Available? Available available - what does Available show?

at this stage I suspect that there is a software reporting module that is not correctly
reporting you total physical ram

I now suggest that you use a ram testing program like memtest or some other ram
testing diagnostics (not the default one that you have used so far)

if I were you I would download a bootable DVD or usb that has diagnostic testing prgs
like Hirens or something similar

don't have - ask some experts here to show you where to download

so looks like you've installed the 32 bit of windows which cant use beyond 4gb

back to fresh install is your only way

problem solved