4gb lol, dream on.
2gb ram will extend the useful life of the Air 2 for a while given the constraints that the Air 3 will have. Everything points (probably) to a higher resolution next iPad, but will Apple increase the ram again? Doubt it with their record. Equally, will the new GPU in the next iPad absolutely, positively be 100% capable of pushing those extra pixels silky smooth all of the time? History suggests not.
I was happy with my old iPad 3 for years, but it wasnt the fine balance of hw that the iPad 4 undeniably was. Same applies to the Air 2 over the Air 1 imo. The Air 2 is essentially "overpowered"* for what it's limited to do currently, and I can only see that changing if/when multi window functionality arrives.
It's all subjective, but standing back I think the best iPad's for longevity and useful life have been 2,4 and Air 2 (admittedly too early to judge the latter). Note; I've previously had 1,3 and an rMini (which is still great), so I'm certainly not beating the "what I've bought has been the best" drum.
All of the above relates specifically to the evolution of the iPad as we know it. A possible iPad Pro could be entirely different, but until it exists speculation re: that is 100% hot air (excuse the pun), as opposed to considered expectations of what we've seen from previous iPads. I know considered speculation isn't very popular, people love and (mystifyingly) fully subscribe to incredibly unlikely rumours, despite the fact, aside from the swift 3->4 transition, iPad evolution has been incredibly predictable. Let's not get like Gene Munster and peddle the same BS 100% accurate(!) rumour that Apple are releasing a physical Apple TV next quarter - 5 years and 20 quarters later, no sign. If it did happen, Munster's insight is still about as clever as saying we're all going to die.
*Overpowered in the sense that the gpu is absolutely smoking for the current res, and the extra ram appears yet to be utilised fully. I don't buy into comparisons with desktop/laptop cpu's, hard to compare like for like when so much tablet software is Fisher Price next to traditional computer applications. Please don't suggest looking at Geekbench scores, yawn.