Fait mathématique: 4209 is the number of conjugacy classes of the alternating group A_{32}.
Culture générale: 4209 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Date historique: July 10th is the day in 138 that Emperor Hadrian dies after a heart failure at Baiae, he is buried at Rome in the Tomb of Hadrian beside his late wife, Vibia Sabina.