On clocks and watches with roman numerals why is 4 usually noted as IIII rather than IV?

When mechanical clocks were first invented, in the 14th century, they were displayed in public places, usually cathedrals. The faces themselves were only ornamental at first, for the early models had no hour or minute hand but merely gonged once for every hour of the day.

Clocks were thus of special value to the common people, who were almost universally illiterate. Most peasants, even in Italy, could not read roman numerals, and they could not subtract. They performed calculations and told time by counting on their fingers. Four slash marks were much easier for them to contend with than "IV", taking one away from five.

Why Do Clocks Run Clockwise? by David Feldman