Just buy 2 x 4GB because 8GB is the best that model gets.
You could gain a lot more benefit from replacing that 160GB 5,400 r.p.m. hard disk with a higher capacity solid state drive.
Go to www.crucial.com and download their memory scanner. It's quite safe to use and it will tell you the exact specification of the RAM and by how much you can expand it. It will also give you a price for it but you don't have to buy from them. It might also mention the SSD upgrade and price that for you as well.