Well first of all, with a 32-bit OS, you're only going to be able to use maybe 3 of the 4GB of RAM, and if you add more, won't change a thing. You'll need to install a 64-bit OS in order to use more RAM.

The next thing you can try is open up your computer, if it's a desktop, and just check how your RAM is arranged. There should be a couple of sticks in there somewhere. See if you have just 2 slots or more slots. If only 2 and both are filled with your existing RAM, you'll likely have to replace both with 4GB modules giving you a total of 8GB. DDR2 I'm expecting, but again, hopefully it'll say in your system specs, or on the side of the actual RAM.
If you have 4 slots for example, and 2 are full, then just buy exactly the same chips as are already in there, and you should be good to go.

EDIT: Sorry, didn't realize it was a laptop... that makes things a bit more complicated ...one reason I prefer a desktop, much easier to upgrade.