That is probably Number Four sunning himself behind the box. You can just barely see his beak on the right. Remember, the two biggest fledglings are the females; the smaller two are the males.
Things happen quickly now as the last of their down feathers give way to adult flight feathers. PBS has this:
Hatchlings are covered with fluffy white down, replaced by feathers in 21 to 35 days. Peregrines first fly 35 to 45 days after hatching, when they have reached adult size. They begin to hunt around 60 to 80 days.
This is a fun time for box watching. There’s lots of flapping about the roof and ledges and of course the feedings continue apace.
One ASRI technician commented during the banding video that, while the parents will urge the fledglings out of the box, they don’t make them leave the area right away.
He suggested that people should keep checking the skies around the top of the Superman building for a few weeks. In that in-between time, all six birds might be seen flying around in the air. But then the adults will assert themselves, and the younguns will have to move out and find their own territories.