I think ipad 3 would be able to run iOS8 as well as ipad 4 (especially since they're selling the ipad 4 again)
They both have the same amount of ram (1GB), just the a5x and a6x are the major differences. The main limiting factor for which devices get what iOS version has historically been because of RAM.
ipad 1 and iphone 4 are both on the A4 processor (and the A4 is extraordinarily slow compared to pretty much everything), yet the ipad 1 is locked to ios 5.1.1, whereas the iphone 4 is on 7.1 now. This is because the ipad 1 only has 256MB of ram, compared to the iphone 4's 512MB.
While the ipad 3 is "slow" it's mainly due to the new graphical effects implemented in iOS 7 (and pushing all those pixels on an A5X), and in iOS 8 I don't see them implementing new graphical effects that change all that much from iOS 7 (see transitioning from iOS 5 to 6). The main killer here will be RAM, each new generation ram usage climbs, yet even the ipad 2 can run iOS 7 "okay" despite the 512MB of ram, because iOS is already very good at managing cpu work.
Now, will the ipad 2 support iOS 8? Well considering they're still selling the original non-retina mini (which is basically the ipad 2), it's hard to say. If it does that will give the ipad 2 a really long life. I want to say that the ipad 2 is the cutoff for ios 8 (as in ipad 2 and non-retina ipad mini, as well as iphone 4 being limited to 7.1), but who knows.