The ultra-fast PCIe SSD in the new Airs will go a long way towards mitigating the effect of swapping slowdowns, so you're probably fine with 4GB for now.
That said, if I were buying a machine I was expecting to use for a few years, I'd be getting the best I could afford (particularly in something I can't upgrade later). It's probably safe to assume that 4GB would be a limiting factor by 2015/2016.
My 15" MBPr fits on an non-reclined-chair airplane tray just fine, so I'm sure you'd be safe with a 13".
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