Based on your post, you have the stick of 4GB RAM (DDR3-1600mhz). I'd suggest you to add the same specification RAM model. Most motherboards like to see RAM pairs installed when using two or more RAM sticks. RAM speeds match with what you have with you and order the same frequency RAM, otherwise the slowest RAM is the default speed for both of them. With the different size of RAM, you are not able to use the double channel function for performance.
Just a suggestion, if you run 2 RAM sticks with same specification; you will be running memory in dual channel which will be a greater increase in performance than running a single stick in single channel.