“However, even though it is now widely accepted that 4 must be written IV, the original and most ancient pattern for Roman numerals wasn’t the same as what we know today. Earliest models did, in fact, use VIIII for 9 (instead of IX) and IIII for 4 (instead of IV). However, these two numerals proved problematic, they were easily confused with III and VIII. Instead of the original additive notation, the Roman numeral system changed to the more familiar subtractive notation. However, this was well after the fall of the Roman Empire.”
“However, even though it is now widely accepted that 4 must be written IV, the original and most ancient pattern for Roman numerals wasn’t the same as what we know today. Earliest models did, in fact, use VIIII for 9 (instead of IX) and IIII for 4 (instead of IV). However, these two numerals proved problematic, they were easily confused with III and VIII. Instead of the original additive notation, the Roman numeral system changed to the more familiar subtractive notation. However, this was well after the fall of the Roman Empire.”
https://monochrome-watches.com/why-do-clocks-and-watches-use-roman-numeral-iiii-instead-of-iv/