Mathematical fact: 4121 is a number whose product of digits is equal to its sum of digits.
Trivia: 4121 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Historical date: April 13th is the day in 1919 that Eugene V. Debs enters prison at the Atlanta Federal Penitentiary in Atlanta, Georgia, for speaking out against the draft during World War I.