There doesn't seem to be a definative answer to this one but http://www.ubr.com/clocks/faq/iiii.html
come up with several suggestions. Some are better than others!!!
I think that IIII on the clock face makes the whole dial look more balanced. The IV in the four place creates a sort of disturbing void. I think it was invented by the Victorians for just this purpose as the Romans never used this form of four, using the IV form that conformed with their number stucture.