Guys I got this RAM, which one would be a good one to pair with it [Picture included]

[Samame]

Hello guys i got this PC a year ago now and it came with 4 GB ram, it is from a Corsair 8gb 2x 4gb kit but it only came with one, I'm not able to buy the other one cuz they only sell the combo, which RAM should I get so I don't mess with the RAM speed or something?

Heres a picture of the RAM

 

[PacaAL]

Get another DDR3 4gb stick. If you have 4 slots knock yourself out with 3 more 4GB sticks. You can get sticks with different GB amounts if your mobo and OS supports that kind of thing.