Can you please provide us with more information? System model? Motherboard model?
It is very possible that your motherboard does indeed support 8GB of ram if you have 1 4GB module in one port. Generally speaking, if one port will support 4GB of RAM so will the other port. You will need to purchase another RAM chip to put into the second slot that is 4GB. This is actually perferred, not only will you get more RAM your system most likely supports Dual Channel Memory and this will increase your system performance as well.
Please let me know you system information along with RAM information and we can get you taken care of.
RAM EXAMPLES: PC3-12800 / 1600 MHZ