For years I've wondered why the number 4 on a Roman numeral watch or clock is – most often than not – represented by IIII and not IV.
Here's an explanation...

Clock faces that are labelled using Roman numerals conventionally show IIII for 4 o'clock and IX for 9 o'clock, using the subtractive principle in one case and not in the other.

There are several suggested explanations for this, several of which may be true...

1 The four-character form IIII creates a visual symmetry with the VIII on the other side, which IV would not.

2 The number of symbols on the clock totals twenty Is, four Vs, and four Xs, so clock makers need only a single mold with five Is, a V, and an X in order to make the correct number of numerals for the clocks, cast four times for each clock...
MV IIII IX
MVI II IIX
MVII III X
MVIII I IX

IIX and one of the IXs can be rearranged or inverted to form XI and XII.

The alternative uses seventeen Is, five Vs, and four Xs, possibly requiring the clock maker to have several different molds.

3 IIII was the preferred way for the ancient Romans to write 4, since they, to a large extent, avoided subtraction.

4 It has been suggested that since IV is the first two letters of IVPITER (Jupiter), the main god of the Romans, it was not appropriate to use IV as a number. (Read The Alphabet by David Sacks for the explanation of why Roman letters lacked J and U.)

5 The I symbol would be the only symbol in the first 4 hours of the clock, the V symbol would only appear in the next 4 hours, and the X symbol only in the last 4 hours. This would add to the clock's radial symmetry.

6 IV is difficult to read upside down and on an angle, particularly at that location on the clock. [Though I see in my image here that VI is upside down.)

7 Louis XIV, king of France, preferred IIII over IV, ordering his clockmakers to produce clocks with IIII and not IV, and thus it has remained.

See wikipedia.org