If you have a 32 bit OS you will not be able to use more than 4GB of RAM. If you have a 64bit os and are showing the wrong amount of ram in bios and os, it may be you have a bad memory slot, incompatible ram or the ram is not seated properly.
I have a 64 bit os running. This ram has been fine for years up until I recently noticed the issue. I have reseated the cpu and ram. I have also flashed bios and reset CMOS. I have also made sure max memory is unchecked in msconfig. I have a dedicated GPU as well.