Your laptop probably came with an earlier version of Mountain Lion (10.8). The latest update, 10.8.5, isn't really a "new" OS, but a bunch of bug fixes, security enhancements and other little improvements.
The new OS, Mavericks 10.9 is due to be released in a month or so. It should run fine on 4Gb of RAM.
However, you can never have too much money or too much RAM.

RAM is "where your computer puts stuff when its working on it".
A good analogy is a kitchen or workshop. Your hard drive is the cupboards where you keep things. When you want to do stuff, you take it out of the cupboards and put it on the work surfaces. (That's RAM.) The more work surfaces you have, the more stuff you can get out of the cupboards and spread around, and still have space to work, before you have to put stuff back.
Memory modules are usually pretty cheap, and they are a user-serviceable part on most Macs, so you could buy them online and install them yourself.