What version of Windows do you have installed (Vista, Windows 7, Windows 8, etc.), and is it the 64-bit or 32-bit version?
If BIOS only shows 4 GB then that's all you have. If you have 8 GB installed and only 4 GB shows up in BIOS, then 4 GB of it is either incompatible or not mounted properly in the DIMM (RAM module socket on the motherboard).