From my local jewellers blog.
Did you know?
Many Rolex owners have wondered, when gazing upon the roman numeral appliques of their watch dial, why the four is transcribed as IIII rather than IV. This IIII is known as the clockmaker’s four, and it is far from being a recent convention. The notation IIII was used by the Ancient Egyptians, Greeks, and later Romans. It was not until the late Middle Ages that IV came into use. When the first clocks were made, between the 13th and early 14th century, IIII was still in common use. Today "the clockmaker’s four" is kept due to the wonderful balance it brings to the dial.