Yes, we all know how exciting 4gb is, lol. I keed I keed. I get what you meant.
So you can probably add it and it will probably work. However, the caveat is that it may only run in single-channel mode, and there is a chance it may not work at all.
The better option would be to just buy a second identical 4gb module (should be $20 or so used) and then you have a total of 8gb. If you need more, then you can get a 'kit' (which is typically a pair of ram designed to run together, but since ddr3 is forgiving it will just be an identical set of two modules). You can get a kit of 2x4gb modules and you'll have a total of 16gb of ram, or you can get a kit of 2x8gb and have a total of 24gb of ram. It really depends on how much ram you need.