How do you use 4GB of RAM?

What's your operating system? 32bit Windows XP only supports 2GB for applications. If you have 4GB the other half is allocated for the kernel.

And btw, virtual machines are a great way to waste resources.

Editing large images in Photoshop is a very, very easy way to burn through lots of RAM. Anything where you work with a large dataset, and it all has to be in memory for processing.

Hundreds of tabs? Firefox can easily chew up 800 MB for me.

I run out of memory all the time, because I'm a programmer dealing with large datasets. I've also run out of memory on a 128 GB machine... but we're working on fixing that.