Fatto generale: 4991 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Fatto matematico: 4991 is a Lucas-Carmichael number.
Trivia: 4991 is an unremarkable number.
Data storica: August 30th is the day in 1974 that a powerful bomb explodes at the Mitsubishi Heavy Industries headquarters in Marunouchi, Tokyo, Japan.