There does not seemed to be a definitive reason why the number four is represented by IIII rather than IV, but it is how the dials on tower clocks have been done for centuries.
Possible reasons include:
- IV was commonly used to notate the Roman god Jupiter (in Latin, Ivpiter), and the numeral IIII was used to differentiate
- Visually, IIII forms better symmetry with the VIII on the other side of the clock; as well as creating radial symmetry in that only I appears in the first four hours, V only appears in the second four hours, and X only appears in the last four hours
- IV is oddly positioned and difficult to read from the normal angle where four appears
- Romans simply preferred IIII as they largely avoided subtraction
For further commentary, please see this document by Don Lathrop, or Wikipedia.
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