For the kind of use you describe, I'm sure that 4GB should be adequate. There's certainly no harm in getting 8gb if you can afford it though. But I have a 2011 MBA with 4GB and run pro applications like Logic and Final Cut Pro with no issues. I would have gone with 8GB if it had been available, but I'm not sure if it would have made much difference.
I went from a 4GB 2008 c2d 15" MBP to the 2011 i5 MBA and the speed difference was quite dramatic. The CPU seems to be almost twice as fast. I'm sure you will see an even bigger difference in the 2012 MBA.
But since it sounds like you might have nagging doubts if you get the 4GB, then why not just get the 8GB? I can't think of a reason not to, unless you just can't afford it.
Really wish I had read this prior to yesterday when I ordered the 4 :/
Just realized this afternoon that it's soldered to the board and not user replaceable... guess I'll return my 4gb and order an 8gb when it arrives.