Why did ancient Romans sometimes use both IIII and IV for 4?

The use of both IIII and IV to represent 4 in ancient Rome reflects the fact that the Romans were inconsistent in their application of additive and subtractive principles. While the subtractive principle—placing a smaller value before a larger one to indicate subtraction—was known and occasionally used in inscriptions, it was little used during the Classical period.12

Instead, the additive method was generally preferred in inscriptions, where symbols were simply grouped together. Consequently, IIII was a common way to express 4, as it followed the standard additive logic of repeating symbols. While IV eventually became the standard notation for 4 in modern usage, the older IIII form persists in specific contexts today, most notably on clock faces.21