4th - We have four, fourteen, forty(drop the u), fourth (don't drop the 'u')
5th - We have five, but then the 'v' become 'f' in fifteen, fifty, fifth ( the 'f' helps pronunciation)
9th - We have nine, nineteen, ninety, but ninth (drop the 'e')
12th - twelve, twelfth (Notice the 'v' becomes 'f')