Maybe they hatched earlier than I thought, because a couple of them were flying a bit today.
When I came home from work, one of them was perched on a rail about 30 feet away from the nest.
Another one was fluttering around a bit near my doorway. He didn't seem comfortable going too far, but he was able to hover and then land on the nest. (That was pretty cool to see.)
A third one was staying in the nest. That one holds his (her?) wings a bit funny--up higher closer to his head. It almost makes him look like an old man with a stoop. I'm wondering if that one is healthy.
When one of the parents came to bring food, Stoopy was the only one I saw stick his head out of the nest. So I can't account for one of the hatchlings. I looked around a bit but didn't find the fourth anywhere nearby on the ground.
Update: I just stepped outside to have a look, and all four hatchlings are back in the nest. They're big enough now that it's pretty crowded.