Just taught my fourth graders Roman numerals. All 4s and 9s are created by a subtraction combination involving a smaller number reducing a larger one. Only i's, X's and C's can reduce another number larger than itself. None can reduce a Roman number more than two spots higher [I can only reduce V (5)and X(10), X can only reduce L (50) and C (100), and C can only reduce D (500) and M (1000)]. V, L and D cannot reduce another numeral. This means that no letter will be used more than three times in a row. 4 is IV (5-1), 9 is IX (10-1), 40 is XL (50-10), 90 is XC (100-10), 400 is CD (500-100) and 900 is CM (1000-100).
After MMMCMXCIX (3999), 4000 is created by writing an IV with a line over it. And so on.
Confused? Yeah, so were they.