There are three reasons why clocks normally display 4 as IIII rather than IV. Firstly, being so much longer, it strikes a better aesthetic balance with the VIII on the other side, as Sddsd says. Secondly, because all the numbers stand at varying angles around the circular clockface, it avoids confusion with VI. Thirdly, there is evidence that IIII was actually an older method of indicating 4 than IV was in Roman times. In other words, there is every reason to use IIII rather than IV.
Having said all that, IV is used on what is perhaps the most famous clockface in the world - that is, the one attached to Big Ben on the British Houses of Parliament!