I typed purge in terminal and got the reply below.
-bash: purge: command not found
Hm, I'm not sure what I'm doing differently, but I just tried it and it works for me. I actually learned that tip here on MacRumors so maybe someone else can shed more light on that.
Anyway, I upgraded to 8gb of RAM, and I can say it really does help a lot. I can now run virtual machines without any sluggishness at all. I'm kind of ridiculously cluttered and keep a lot of windows/apps/tabs open, so I still do end up using swap space. But based off your activity monitors screenshot, you don't seem like you would ever have that problem. Not to mention, even if I do swap a little, I don't notice a slowdown.