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If you include 0000, ten thousand unique four digit codes are possible.
A four digit lock can have as it's combination any number between 0000 and 9999, or a total of 10,000 variations. Good luck guessing one.
If a leading zero is allowed... The first digit can be 0-9, ten possibilities. The second digit can be 0-9, ten possibilities. The third digit can be 0-9, ten possibilities. The fourth digit can be 3-9, 7 possibilities. So there are 10x10x10x7 = 7000 such codes.
Digit four represents ones.
There is 90 four digit palindromes.