Only RAM sold in kits are guaranteed to work. The controllers are super finicky and need to work well together, so the manufacturer tests them prior to purchase. For that reason, even the exact same RAM (brand, speeds, voltages, size) may not work if it is not sold in a kit.
You are trying to save money by buying another 4gb stick, but consider what you will do if they don't work together. You will be in the exact same boat except with two sticks instead of one.