[Betty]

from a wiki

Clock faces that are labeled using Roman numerals conventionally show IIII for four o'clock and IX for nine o'clock, using the subtractive principle in one case and not the other. There are many suggested explanations for this, several of which may be true:
The four-character form IIII creates a visual symmetry with the VIII on the other side, which IV would not (with the exception of square faced watches and clocks, where the opposite number is X).
With IIII, the number of symbols on the clock totals twenty I's, four V's, and four X's, so clock makers need only a single mold with a V, five I's, and an X in order to make the correct number of numerals for their clocks: VIIIIIX. This is cast four times for each clock and the twelve required numerals are separated:
V IIII IX
VI II IIX
VII III X
VIII I IX
The IIX and one of the IX’s are rotated 180° to form XI and XII. The alternative with IV uses seventeen I's, five V's, and four X's, requiring the clock maker to have several different molds.
IIII was the preferred way for the ancient Romans to write four, since they to a large extent avoided subtraction. (However, nine is written IX on clocks, and the IV notation is ordinal rather than subtractive in origin.)
As noted above, 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.
Only the I symbol would be seen in the first four hours of the clock, the V symbol would only appear in the next four hours, and the X symbol only in the last four hours. This would add to the clock's radial symmetry.
IV is difficult to read upside down and on an angle, particularly at that location on the clock.
Louis XIV, king of France, who preferred IIII over IV, ordered his clockmakers to produce clocks with IIII and not IV, and thus it has remained.

The explanation about using one mold seems like a good one.