On iPad, there are some useful multitasking gestures such as swipe between applications.
How do I enable multitasking gestures (a la iPad) on iPhone 4?
iOS 4.3 beta had this feature. I used it and it wasn't that nice. The all of your fingers toward the middle brings you home gesture was difficult to use at best.
As of right now, you have to jailbreak your device and install the Zephyr app from the BigBoss default repo.
It lets you configure how it works, which is a nice bonus.
Unfortunately Apple hasn't given us these features on the iPhone, and I'm assuming it is because the screen is small enough that it would be a little awkward to fit four fingers on the screen.
Maybe iOS 6?