why number four in roman clocks is usually be written as IIII instead of IV

The reason why the number four is represented as "IIII" rather than "IV" on some ancient Roman clocks and sundials is due to a misunderstanding or error that has been perpetuated. The correct Roman numeral for four is indeed "IV," which stands for "1 5," indicating one less than five, in the same way that "V" or five is understood.

However, "IIII" was mistakenly used occasionally in some historical contexts, possibly due to:

1. Aesthetic Consistency: Using uniform markings made the sundial easier to create, given the circular design.
2. Practical Errors: These errors might have originated from craftsmen who were not entirely familiar with the rules of Roman numerals.
3. Ancient Uncia Use: It's sometimes suggested that the practice may stem from an "uncia" or "twelfth" part division system, where "IIII" could be seen contextually similar as the first four divisions of twelve hours.

It's important to note that while these clocks exist and use "IIII", this representation is incorrect by the conventions of Roman numerals. Generally, most Roman inscriptions and educated references use "IV" to represent the number four. Thus, the "IIII" representation is a historical curiosity rather than a normative use of Roman numerals.