[BugsJustFindMe]

The third image they show uses IV.

[al_borland]

As much as the IIII has bothered me over the years, actually seeing IV on a clock face makes me think IIII is the right choice.

That said, I always go with Arabic numerals, so it's a moot point for me, practically speaking.

[timbit42]

I've seen many clocks with either.