The MacRumors site is exactly that, however on this I think they have a good lead. Whether Apple will continue with (say) 11″ and 13″ Airs with a new 12″ retina Air or whether they reduce the choice of screen size it’s seems very likely thee will be a new retina Air quite soon.
As for waiting for technology, that’s always a bit pointless and there will always be something better along “soon”. The plus with Apple is kit has good second hand value so you can sell and buy the new release each year or two.
In terms of the range the Air and Maczbook Pro are quite similar in weight really since the Pro slimmed down, the Air is super thin and also the CPU is deliberately slowed so it uses less power and thus has longer battery life.
OP wait for the new Air to decide and get 8gb RAM and 256SSD – storage you can add externally in the future if you need it.