The S4 thing siphons the fuel from the left lobe of your tank to the right lobe. When you drain your fuel down below the center rib, you then only draw from the right side of the tank. You can manually lean your bike over to the right and slosh fuel from the left side to the right, but it's inefficient and a pain. S4 does this for you...automatically. I don't know exactly how much fuel remains permanently trapped, but if you're running out and a gas station is 10 miles away, it sure beats walking.
The Adv GS solves this problem with a lobe to lobe cross-over tube built into the tank. Unfortunately, the regular GS's don't have this. The S4 solves this.
[i]"I mean, really ... when you get right down to it, does a man need anything much more than bacon