Both forms are accepted, but the Roman version that appeared first is the one with the numeral IIII.
The principle of subtraction, which underlies the IV form (IV=V – I), is a later concept. But then why do some watches still use the older form? Explanations vary.
One hypothesis is that using the IIII makes the watch more aesthetically balanced. Just look at the dial: the first four hours are represented by the numeral I (I, II, III, IIII), the next four use the V (V, VI, VII, VIII) and the remaining hours the X (IX, X, XI, XII). More symmetrical than that, impossible!
Another hypothesis for the continued use of IIII is of religious origin. The name of the Roman god Jupiter is spelled in Latin as IVUPITER. That is, in order not to use the initials of the god “in vain”, some Romans would have chosen to keep the old form IIII.
Here in Brazil, a good example of a clock that uses the numeral IIII is the one at Estação da Luz, in the city of São Paulo.