>> a browser with several tabs open

I almost never have "several" tabs open. It's usually ~15 tabs on my main browser (Chrome), and often more than 30. And also an additional 5 tabs in Firefox, Safari, and several versions of IE.

Right now, web browsers are using over 2GB of memory. This is fairly typical from the web developers that I work with. So perhaps it depends on what your software targets. But I would argue if you're building web apps, then 4GB RAM is by no means excessive.