If one starts at 12 [XII] and goes down to 1 [I], while using IIII for 4 the count is X's four, V's four & I's twenty. So maybe there was some reason for wanting these all to be even numbers? If the standard IV was used for 4 the count would be five V's and seventeen I's. Trouble with this guess is I really can't think of a reason that it would matter.