Toggle between applications via four finger gesture?
The best way to do this is with one of the more obscure settings in BetterTouchTool
It works like this:
 A three fingered right swipe brings up the application switcher. Lift one finger to covert it into a two finger swipe and slide to whichever application you want.

It becomes an incredible natural and fluid gesture The beauty of the gesture is that it works anywhere on the screen and you don't have to position the mouse over the icons.


In BTT assign the three finger swipe right to the application switcher.
In BTT Action Settings -> Stuff(App Switcher) -> Use special application switcher


This answers the problem that bisko and senseful had above and avoids the last click or the necessity to be over the icon.

Single best gesture in my 'gesture vocabulary' and I have gestures for everything!