This is certainly one of the questions asked most frequently at the German Clock Museum. A whole variety of different and sometimes very creative explanations are circulating about this issue. But the truth is probably much simpler.

The short answer is: ’IIII’ was the common way of writing the number 4.

A look at the history of Roman numerals might cast light on the reason why: Roman numerals evolved more than 2500 years ago from counting with one’s fingers and with simple marks. Such marks were used everywhere in day-to-day life where counts were recorded with the help of notches or tallies.
A vivid example of the principle of recording numbers by means of notches are the so-called Wässertesseln from the Swiss canton of Valais. These wooden sticks with notches registered irrigation water titles. The photo clearly shows notches cut into the sticks, e.g. IIII and also V.  

We can find countless evidence of the use of the spelling ‘IIII’ from the Middle Ages and the modern era: stonemasons’ marks, numerations on wooden beams (see photo) used by carpenters, page numbers on hand-written parchments and in book printing, tally sticks, tally sheets used in everyday life, and not least year numbers.
In this way of spelling, up to four characters representing ‘1’ were placed in a row, that is, ‘IIII’ (4), as well as ‘VIIII’ (9) or ‘XVIIII’ (19).
In contrast, the spelling ‘IV’ uses fewer characters. It has also been used since Roman times, but rarely ever applied in everyday life prior to about 1500. Any self-experiment shows that calculating 5-1 = 4 as applied in the spelling ‘IV’ requires significantly more mathematical thinking than the addition 1+1+1+1 = 4, which emulates the way we count.

How ‘IIII’ made it onto clock dials

The numbers on clock dials were made by blacksmiths, goldsmiths and clockmakers. They used the spelling ‘IIII’, which was common in the world of crafts. And ‘IIII’ has an additional, very concrete advantage: It prevents confusion between IV (4) and VI (6). This may be helpful when the numbers are placed around the circle traced by the hands in “petal style“ (that is, the numbers 5 to 7 are upside down).

The fact that ‘IIII’ is still the number of choice for current dials may be due to another aspect: the symmetrical balance between IIII and VIII. This assumption is supported by the fact that the number 9 is spelled ‘IX’ on dials – which is graphically about as long as ‘III’.

Why do many viewers consider ‘IIII’ to be wrong?

For more than five centuries, we have been using the Arabic numerals 1,2,3,4 in our everyday lives. They simplified addition and multiplication to such an extent that nowadays we can even learn these operations in elementary school.

Ever since, Roman numerals have been reserved to some selected situations: page numbers of valuable book editions or inscriptions on memorials and tombstones. For these carefully prepared applications, it is common to apply the subtractive way of spelling with the elegant symbol ‘IV’. This is also the variant that has been taught in school and it is the only spelling presented on German-language educational servers. But history shows that it has not always been like this!