Let's start by running msconfig.exe. On the start tab click extended options and uncheck the maximum memory option, if it's set. If that's not set, run resmon.exe and see if the missing 4GB are "hardware reserved". That means simple, that it's a faulty stick and windows could not write to it (bios only reads the spd config but does not check the ram itself).
Thanks, you're right!
The maximum memory option on Boot tab was set to 2048 MB.
After unchecking that and restarting the OS, I got 5.48 GB usable now. I'm assuming that the other 0.52 GB is used by Radeon HD 6520M integrated graphics.