I was wrong and needed the 4 inch. Thankfully Home Depot is great on returns and the 4" was $90 cheaper. They are very happy with the new door and I learned a bit in the process. Bid job so that covered my time on the return and got a door and storm door out of it. Their old door has an xl dog door in it. That alone is like $70 so I came out ahead.

I kind of surprise myself sometimes with what I am able to do. One thing to do it for my own house but there is a different level when doing for someone else that needs to be cleaner and more professional looking.