Memory Usage for MacBook Pro 13"

So I finally went to the Apple Store and got my very first Mac, a MacBook Pro 13" (special thanks to everyone on here who helped me make the decision!).

Anyways, I got the baseline configuration that came with 4 GB of memory. Playing with the Activity Monitor, I noticed that after awhile of using the computer, the green "free" memory piece of the pie becomes progressively smaller until it's almost gone. Does that mean I have no more free memory left? But I'm not running a lot of programs at once. Just Safari and iTunes, maybe Pages too.

My MBP doesn't feel slow, but it always looks like it's out of memory. Any thoughts?