The year 4 (IV) was a leap year which started on a Tuesday, according to the Julian calendar. According to the Gregorian calendar, it started on a Thursday. At the time, it was known as the "Year of the Consulship of Catus and Saturninus". It was called the fourth year since the early medieval period, when the Anno Domini calendar era became the usual method in Europe for naming years. It was the 4th year of the 1st century.