Let's just resurrect this thread in a few years and change the 4s to 8s and 8s to 16s. It will save time and all of the advice will be exactly the same.
A few years ago it was worse when we only had 1 and were trying to figure out if we should go to 2 or 4!
The big difference is that now the RAM is soldered on.
Personally, I'm probably just going to stick with 4. I know I've kept going back and forth on it. But based on my projected usage, 4 is going to be plenty (my desktop at home only has 3). Also, while the Air will be my primary machine, it won't be my only machine.
Plus with all of the improvements Apple is making to their computers & OS X, the average user won't notice much of a difference. OS X has always been good at making do with RAM and it's only getting better. And, in my opinion, Apple doesn't sell computers with inadequate RAM
for average use: meaning that for web browsing, music, movies, documents & light gaming, the base RAM is sufficient. "Power-users" (VM, Audio/Video Production, Graphics) would likely upgrade from the base RAM.