The guy started at a new preschool on Tuesday. There have somehow been zero transition issues. (Hopefully, I'm not speaking too soon.)
It probably helps that he knows a guy there from his old daycare. He just jumped right into playing with him as soon as that guy walked in the door. He's taken well to the teachers, and the space isn't as crowded as the old one, resulting in a much calmer atmosphere.
After school, he played with a friend he met that day, who happened to go to the same playground we went to. (Four-year-olds actually can make friends in a day and have genuinely good times with each other.) They and another kid there see-sawed, went on a tire swing, and chased each other. I had a nice chat with his dad. Preschool victory!