Balance is the key point as stated above. 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 - though almost upside-down - that the 4 is IV and not IIII.
In Roman times either form - IIII/IV - was used apparently, but legend has it that the IIII tradition for clocks began when a French clockmaker made a timepiece for the king. The latter decided that IV was wrong and - when the clockmaker insisted it was right, he was reminded that the king was never - ever - wrong! So the IIII version was preferred thereafter.