This is one of our most popular questions and there doesn't appear to be a definitive answer!
The style IV is relative modern form of the Roman numeral for 4. IIII was used until 'modern times'
There are several sites that have attempted to analyse the pedigree. Some claim royal heritage in either England or France. Others base it on symmetry of the clock face.
Big Ben (the famous London landmark) uses IV rather than the more usual IIII found on most clocks. Check this new Big Ben site.