If it means anything, I have last years Air with 4 GB, and this years with 8 GB.
If you plan on keeping the Air for more than a year or so, definitely wait for the 8 GB. You will never regret it.
You may not notice a whole lot of reason today, but it will pay off immensely in a year or so. Your needs will grow, the software you use then will require more, and the OS will take a bit more. Since you can never upgrade an Air, and the price is so reasonable, there is no logical reason to deprive yourself the purchase.
Have patience, buy the 8.