Clock-faces with Roman numerals usually represent 4 by IIII rather than IV. This is generally taken to be so that the four elements of the number better balances the four elements of VIII for 8 on the same level at the other side, as
Octavius says.
However, the clock-faces on the �Big Ben' tower at the Palace of Westminster - probably the best-known such clock-face in the entire world - actually uses IV for 4! Click
here for a close-up picture of it. It shows clearly - though almost upside-down - that the 4 is IV and not IIII.