I'm pretty sure I know the answer to your question (although I could be wrong). Are you using a 32 bit Operating System? To find out right click on my computer and click properties. 32 bit OS can only handle less than 4gb of RAM. If so then you will have to upgrade to 64 bit OS. There is normally another DVD for 64 bit with a retail version of windows.
defiantly the os ill give a explanation for some reputation ok so here it is
when the 8088 they didnt think of the future and thought we will never need more than 2 gbs of ram so they didnt develop 64 bit processors
and that means that the memory controllers on the cpu only can address 4 gbytes of ram and but a bit of that is taken up by io space which leaves 3.8 gbytes of ram . even though 64 bit processors have been around for yonks there a some people still rocking a 32 bit now for microsoft to win the battle against of os's it hasto be widly available so they make a 32 bit version but that only has support for 32 bit cpus so it only can see 3.88 gbytes of ram
First gues is you have a 32bit OS, which 4gb is max, (about 3.8 usable), 2nd guess is you are mixing sticks and if it shows dual channel, then it's only using 2 of the 4GB stick or it's not using the 2GB stick at all, if this is the case can try raising the DRAM voltage about + 0.06 and try that, otherwise will need the model # of the sticks to see if we can get them to play together.
Sorry, but 64bit CPUs can also run the 32bit OS's and have been able to since day 1...You are NOT tied to a 32bit CPU to run a 32bit OS, though a 32bit CPU can not run a 64bit OS