Its just to make it look better symetrically, as the IIII balances the VIII at the same place on the other side. However there are some old clock faces that do use IV - Its the choice of the clockmaker.
Similarly, I always wondered why the number eight was written as "VIII". It seemed like "IIX" would have been neater, but as Domnai said, it's probably just in case you get them muddled up.