With the lack of upgrades in the mini3 and the
hot-glued touchID (ifixit.com) I'd say that a mini4 replacing the mini3 so soon means they missed some deadlines on the mini4 and pushed out a "rush job" mini3 with touchID just to get touchID in there. Could always just be a rumor, no more, though...
If you add in 8.0.1, OS X wifi issues, so-called bendy iPhones, and not-so-scratch-resistant-that-there-is-doubt-it-is-gorrilla-glass screens I'd say Apple is really under some severe timeline pressures. Whether that pressure makes diamonds or crushes the company has yet to be seen. I think they are going to have to space out some development here soon. My guess is there will be a sizable shift in hardware/software release dates right around when the move to the new campus takes place that will be blamed solely on the move...