I'm really impressed that you and your wife are staying chill about that. My belief was similar, but even as our daughter was approaching four we were wondering more and more when that kid would poop on the potty. Which Costco box of diapers would be our last?

Then, just as you describe, one day she just started pooping on the potty and has barely even had an accident since. We're still using those Costco diapers at night, though. May as well get our money's worth.