I recently got a watch with Roman numerals on the face. It, like virtually any other clock or timepiece I've seen with Roman numerals, used "IIII" to designate the equivalent of the Arabic numeral "4". Yet in any other context, "IV" is the accepted Roman numeral for 4. Why the difference?
As it turns out, our descriptions of the 1, 2, 3 system as Arabic and the I, II, III system as Roman are somewhat inaccurate. The former originated in India and arrived in the West via Arabia. The latter came to the classical Romans through the Etruscans and Greeks, and was slightly modified by medieval Europeans. One of those modifications is explained in Oxford University Press' A Dictionary of Weights, Measures, and Units:1