Original MacBook Air Capable of 4 Finger Gestures Through Software Update
I can confirm the four finger gesture is possible on pre-October 2008 laptops, and I currently have four finger Expos and Application Switching working on my January 2008 MacBook Air.
This method is very much a "hack" at this time and is certainly not recommended for the casual user, but does suggest that Apple could enable the four-finger gestures on early 2008 MacBook Pros and MacBook Airs through a simple software update.
When Apple introduced the new notebooks in October, they introduced a new four-finger gesture that could be used to invoke Expose. It hadn't been clear if the four-finger gesture would be possible on existing multi-touch trackpads which have shipped in the MacBook Air and MacBook Pro since early 2008.
Update: First-generation MacBook Air is now available at a large discount.