You have 6 slots for ram , that means it's a triple channel motherboard and in some cases to have 4,8,16gb of ram to work you have to fill all the white slots and the one black slot closest to the cpu socket. Right now you have 4 sticks of 1gb each and to get 8gb to work you need a set of 4 sticks at 2gb each put in the same slots that the ones you have now. You also have the option of getting a 12gb set, 3sticks of 4gb each and they would go into the white slots.
When I say white and black slots I'm referring to the color of the snap connectors at the end or each slot. I just mention the 12gb option in case you have a need for more then the 8gb as you didn't mention the reason for the upgrade.