4

Year 4 (IV) was a common year starting on Wednesday or a leap year starting on Tuesday (link will display the full calendar) of the Julian calendar (the sources differ, see leap year error for further information) and a leap year starting on Tuesday of the Proleptic Julian calendar. At the time, it was known as the Year of the Consulship of Catus and Saturninus (or, less frequently, year 757 Ab urbe condita). The denomination 4 for this year has been used since the early medieval period, when the Anno Domini calendar era became the prevalent method in Europe for naming years.

https://en.wikipedia.org/wiki/4