The babies have fledged! This means that they are off the nest, though perhaps nearby, hiding in the ground-cover. In a few days they will learn to fly.
Carolina wrens develop very rapidly, and it was clear to us that the nest was getting crowded. The baby birds would crowd back and lay low when we were nearby, obviously having progressed from the early days when they simply opened their beaks wide without shrinking away.
We hope all four babies will make an appearance on the patio in coming days.