The whole point of the iOS interface is that each app has its own mechanisms for accomplishing things that are optimized for that app alone, and the device is just a container that enables each app to do this.
So when you're in the web browsing your app, the experience is that you're using a device that has been made for web browsing. When you're in a calculator app, you're using a device that was built to behave like a calculator.
There are certain standards and techniques that all apps adopt when they handle similar tasks (like swipe to delete a row in a list or pinching to zoom and 2-fingered drag to pan) but those are all implemented by that specific app.
If the app decides that a 4-fingered swipe is the natural way of representing 'go to previous page' then thats what it will use.
Now, with a system-wide gesture, the container is peeking in through the application's interface and putting its stamp.
Until now, this was the function of the home button which was clearly outside the app's interface. Looks like that's changing now.