You start with 0001 and count your way up to 9999.
There's a good reason that they're not listed here for you. There are quite
a goodly number of them. Let's figure out how many there are, shall we ?
.0001 to .9999 . . . . . 9,999 numbers
1.001 to 9.999 . . . . . 8,999 numbers
10.01 to 99.99 . . . . . 8,999 numbers
100.1 to 999.9 . . . . . 8,999 numbers
1000 to 9999 . . . . . . 9,000 numbers
Total . . . . . 45,996 numbers that require 4 digits
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The answer is 3,584. The first digit of the four-digit number can be one of seven figures: 1,2,4,5,7,8 or 9 (if it is 0, then you don't have a four-digit number!) Each of the remaining digits can be one of eight figures: 1,2,4,5,7,8,9 or 0. The number of possible combinations is therefore 7 x 8 x 8 x 8 = 3,584.
The smallest four-digit number not using zero is 1000. In a four-digit number, the first digit cannot be zero, so the smallest possible value for the first digit is 1. The remaining three digits can be any number from 0-9, so the smallest possible four-digit number is 1000.
9999
If you include 0000, ten thousand unique four digit codes are possible.
9,876