Are you using a gaming laptop from Alienware, MSI, Asus, Sager or are you using a cheapie that you purchased from Best Buy? If you're using a cheapie, go get yourself some regular Micron/Crucial PC3-12800 and call it a day as performance modules won't make a difference in how it performs.
You only have two DIMM slots to use, and one is currently in use with a 4GB module sitting in it. I'm guessing that this laptop will be limited to 8GB, but you have not posted what kind of laptop you are using. Without knowing that, we can't make a determination. All I can say right now, is that you are limited to dual-channel RAM kits.