In a way, the higher the RAM memory is the more applications you can running at the same time. You can save the hassle to shut down others heavy duty applications before you launch another heavy duty applications.

Nowdays, C2D processor already can handle multi-tasking very well. Basically RAM allow to pushing the multi-tasking in speed performance.

Some graphic applications can already takes away 100mb once launch a new work area. Depends on the file size & resolution somehow.

And the OS might not require full 4GB ram. Example XP will required max to 3.25GB RAM (if no mistaken).