A couple things first, it's highly recommend that you duplicate as close to possible the existing 4GB stick with the SAME: Frequency, CAS Timings, and especially the voltage. Further, MATCHING KITS are the best solution with the least problems; RAM is cheap now. IF possible purchase the SAME existing: Brand and Stick (Part Number) as is already installed.
To determine the Part Number run CPU-z and look at the 'SPD' tab, there it will list the Part Number; it's also should be printed (label) on the RAM installed. If this is an OEM then most likely it will be difficult to find and more than likely from an OEM source it's Samsung.
Next, install the 2nd stick in the SAME color DIMM slot as the already installed 1st stick. You manual should list the correct DIMM slots to obtain Dual Channel.
Examples of Dual Channel; this depends entirely on the MOBO:
CPU: | 4GB | 4GB | blank | blank |
CPU: | blank | 4GB | blank | 4GB |
CPU-z - http://www.cpuid.com/softwares/cpu-z.html
Part Number & Mfg: