Apple doesn't really follow historical models anymore - also historically Apple didn't sell iOS devices with last years processor (EG the Touch 5 and Mini 1).
If the iPod Touch 5th gen gets it, then the 4S will be able to run it. Also the Mini 1 is also still for sale and in widespread use, and it is identical to the iPad 2(in performance related specs).
True. If the iPad 2 and 4s do get iOS 9, it's really only by grace of the iPad mini, which Apple probably feels obligated to continue supporting since it's still fairly new.
I sometimes wonder if Apple did the right thing in releasing all these low-spec variants. Sure, they help cover more price points, but in the long run, it seems to screw up with Apple's software update plans.