People often contact me to suggest improvements to the Livius site – on average, about two or three every day. One of the suggestions I have most often received, is that the names of the legions IIII Scythica, IIII Flavia Felix, and IIII Macedonica ought to be written like IV Scythica, IV Flavia Felix, and IV Macedonica. And VIIII Hispana is supposed to be IX Hispana.
The system that we use to write Roman numerals, however, was not used by the Romans themselves. I have never checked it, but I suppose it is a Renaissance invention. What I did check, on the other hand, was a number of inscriptions that I saw during a holiday in Germany, in which I visited the Saalburg, the Mainzer Steinhalle, the Lapidarium of Stuttgart, and the Limes Museum in Aalen. The Romans had a clear preference for IIII and VIIII; but that may be just the epigraphic habit of Germania Superior. Anyhow, this is why I prefer IIII to IV, VIIII to IX, and XIIII to XIV.
Perhaps, aesthetical considerations determined what the sculptor used: that, at least, appears to be behind the XIIX on the famous cenotaph of Marcus Caelius.