There are two different versions of this model - you will want to remove the base cover and check to see whifch you have.
One version has a single memory socket and takes 8G max (you replace your 4 with an 8 and that's it).
The other version has two sockets and can take up to 2X8 or 16 G RAM.
Did you upgrade your RAM? Hows the system performance post upgrade? Any issues being faced? i have a similar model and looking at one 4 GB (Default that came with the laptop) + one 8 GB ( additional i plan to add). Do let me know if you are facing any challanges with the system post an upgrade.