JJHJIn addition to @MrJamesDuffy's link, I'll add a few more details. Some people think that the IIII balances out the VII across the dial. Also, the IV and VI in close proximity can be a bit confusing. Here's the watchmaker's four on another watch that we have coming up. :)
JJHJFor me, the balance is more with the VIII and XII. I've seen dial layouts with the "proper" Roman numeral IV to indicate the four and that entire right side just looks wimpy, for lack of a better term. Again, I had a problem with IIII at first but my bias has reversed and now IV just looks unsettling on a watch or clock face.