Fait mathématique: 4821 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Culture générale: 4821 is an unremarkable number.
Date historique: March 13th is the day in 1964 that American Kitty Genovese is murdered, reportedly in view of neighbors who did nothing to help her, prompting research into the bystander effect.