4th stage would have to be controlled by the furnace timer. The Vision Pro would control 2 stages of the heat pump, the 3rd stage would fire the furnace. On the XV90, there's an adjustment on how fast you want it to switch to high. Set it at 15 minutes.
With the price of gas these days, I'd go for dual fuel. I doubt there is any payback for the 16i though. Friend of mine is building an extremely tight new house. She asked about the 16 and even buying at 1/2 price (she's an dealer) I can't see that paying back. With the 14 SEER on the variable speed 90, she can get 15 SEER and mid 8s on the HSPF for so much less bucks.
Today's stuff is unreliable, buy an extended warranty on whatever you do.
That sounds like a lot of cooling for such a well insulated house. Are they sure on that load? That much glass?
I have a heat pump and XV80. I use all 3 stages controlled by the stat though the Vision Pro won't do it.