Snow Leopard to Bring Support for 4 Fingers Gesture on MacBook Air

According to Macrumors and confirming an information released one month ago), the last build of Snow Leopard will bring support for 4 fingers gesture on the first MacBook Air and MacBook Pro Penryn. Indeed, the trackpad on those models were already ready to support such technology.
For those of you who would like to make use of it without having to wait still Snow Leopard is released, the procedure are described hereafter for the first MacBook Air and MacBook Pro Penryn.