doesn't an I before an X mean 9?
I The numeral one. II is two, III is three. You seldom see IIII as 4, since IV can also mean 4, plus its shorter to write.
V The numeral 5. IV is 4, VI is 6, VII is 7, VIII is 8.
X The numeral 10. IX is 9, XI is 11, etc.
L The numeral 50. XL would be 40.
C The numeral 100. Think of Century having a hundred years. C is short for the Latin word Centum, but that's not very easy to remember.
D The numeral 500.
M The numeral 1000.