Traditionally purple, the four ball occupies an otherwise unremarkable place among the solids. This particular example has one conspicuous distinction: its numeral is rendered in a different typeface from every other ball in the set. It is almost certainly a replacement.

No one remembers exactly when the original four disappeared, or under what circumstances this one arrived. The others accepted it without much ceremony. It still occasionally feels like an outsider in family photographs, but after enough racks, collisions, and nights together, provenance begins to matter less. The set took it in. It belongs here now.