Why Do Clocks Use the Wrong Roman Numeral for Four?

I never noticed it until the other day when Pat Sajak pointed it out to me on Wheel of Fortune. It is something I have looked at for nearly 50 years and never noticed before. I am talking about our clocks and the roman numerals on them. Most all of them have a mistake on them and I never saw it until now.

If you look at a clock that has roman numerals on it, you will notice that the numeral for 4 is wrong. Four should be written IV but on most clocks it is represented with an IIII instead. Why? I have no idea. I now look at my clock and think it looks like a picket fence instead of a four. Hey everybody it's fence:30! I don't get it. Some people tried to explain it here but I still don't get it. Did you ever notice this before?