I would say go for 8GB purely for theft that it will future proof the MacBook. I have some good knowledge of the Mac and Mac OS in general and the first I will say is that, with the MacBook Air, the RAM you get with it when you buy it is the RAM you'll be stuck with for life. The RAM is soldered to the motherboard of the MacBook, fused into the DIMM slots so you can't upgrade it. Apple have tried to remedy the situation for low memory(4GB and below) systems with memory compression introduced with Mavericks but its best not to play with fire. Your usage patterns may be low demand at the moment but a, those can and will change and b, the apps you use at the moment will be upgraded and become more feature rich, demanding more RAM. DONT cheap out on the RAM.
My advice: Don't get the Air, get the 13 inch Retina MacBook Pro with 8GB RAM and a 128GB SSD and buy an external hard drive. The Pro has a much superior display, faster CPU(none of that ULV crap) and delivers HDMI output as a bonus too. If none of that matters to you, then just get the Air with 256GB SSD and 8GB RAM, make sure it is the 13 inch version though