It's DDR3 on 800 MHz or DDR3 on 1600 MT/s (double data rate of 800) .Read on the stick and tell me the frequency on MHz.It is pretty risky to buy additional RAM because it can cause incompatibilities with the other stick. Also tell the model and the Manufacturer of your stick
Download this : http://www.hwinfo.com/download.php - and tell me or take a screenshot and post it here with all the Info about your specs and everything
Yes you have the possibility to Upgrade your RAM to 8 GB but I recommend you to buy a new kit 2x4 Gb at 1600 MHz.
You need to open your Laptop and see the exact model and manufacturer if you wanna put one more stick of 4 Gb