4

The year 4 (in the proleptic Julian calendar) started on Tuesday, January 1, and ended on Wednesday, December 31. It was (proleptically) a leap year.

Calendar with holidays
Holidays:
 * New Year's Day: Tuesday, January 1
 * Leap Day: Friday, February 29
 * Halloween: Friday, October 31
 * New Year's Eve: Wednesday, December 31