maybe the 4gb ram is reserved for your graphiccard.
you will see this settings in the bios.
reinstall your program - it can help
It sounds like you may a faulty DIMM (stick) in the notebook. Power down and unplug the notebook. Remove one of the memory sticks and power on again. Does it still detect 4 Gb or is it down to 0? Return the first DIMM to the notebook and repeat the process with the second DIMM.
Once you have isolated which DIMM is being detected and which is not, remove the non detected DIMM from the notebook and swap the detected DIMM into non detect DIMM's slot on the motherboard. Power on the system. Is it still detected? If so, then the DIMM you removed is faulty. If not, the slot on the motherboard is likely the issue.
It's possible that during this process you'll see 8 Gb of RAM again, in which case one of the DIMMs may simply not have been seated properly.