According to Wikipedia
http://en.wikipedia.org/wiki/Roman_numerals#IIII_or_IV.3F
'The notation of Roman numerals has varied through the centuries. Originally, it was common to use IIII to represent "four", because IV represented the pagan god Jupiter. The subtractive notation (which uses IV instead of IIII) has become universally used only in modern times. For example, Forme of Cury, a manuscript from 1390, uses IX for "nine", but IIII for "four". Another document in the same manuscript, from 1381, uses IV and IX. A third document in the same manuscript uses both IIII and IV, and IX. Constructions such as IIIII for "five", IIX for "eight" or VV for "ten" have also been discovered. Subtractive notation arose from regular Latin usage: the number "18" was duodeviginti or