If so, then that would most like what is using the other amount of RAM. Integrated graphics will have a portion of RAM dedicated for it's as set in your BIOS.
In your Task Manager screenshot, it shows that 2GB is reserved for hardware.
Even if you don't use integrated graphics, if you motherboard includes that feature, then double check your BIOS/UEFI settings to see if any RAM was set to be used by graphics or any other hardware.
In addition, double check to make sure that the Maximum memory box is unchecked like in step 4 of the tutorial below.
The situation you describe is usually the result of bent cpu pins, or a poorly seated processor. The fix seems to be to pull and re-seat the processor, carefully straightening any bent pins while you have it out. Check this out: