[JdeBP]

Even thinking of this as regional variations developing is superficial and wrong. There likely was not a single central standard to splinter in the first place. Framing this in terms of modern concepts like centralized government-backed standards that get communicated out is a mistake.

The reality is far more likely that the notations IIII and IV were equivalent, to any numerate person of the era; and if asked about what the difference is their response would likely be the same as a modern person being asked about the two glyphs for "a" and "g" in many English alphabet typefaces (or even the various open and closed glyphs that Indian numeral "4" can have). They are so used to the forms that they don't even register a difference that is glaring to outsiders, let alone consider one to be "wrong" and the other "right".