It will work just not as dual channel memory. Dual channel adds 5-7% performance over single channel. So a single 4GB stick is already running in single channel mode so adding a second 2GB stick does not lose you anything in overall performance and helps you by adding more RAM. The best case scenario is 2 matched sticks so the motherboard can run in dual channel mode but as I said the performance loss is not huge. Just 5-7%.