If 4GB of RAM isn't enough, then upgrade it. We can't blindly tell you whether it's worth it for you. The way I use my 13" MacBook Pro, 4GB wasn't enough, and maxing it at 8GB made a big difference.
What are you using it for? If it is just the usual browsing, occassional game and the like, there isn't any point. Any intensive media work, I would definitely add more.