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There are many thousands of possible answers. 11, 12, 78 and 99 is one possible quartet.
There are 9 x 10 x 10 x 10, or 9000 four-digit number that can be formed if the leading digit cannot be zero.
Any pair of digits (not including 0), can be used to generate 14 four-digit numbers. If one of the digits is 0, only seven will start with a non-zero digit.
It is the set of all numbers excluding zero.
If you exclude numbers starting with zero then the first digit must be between 1 and 9 (i.e. 9 combinations). The remaining 9 digits can be any value between 0 and 9 (i.e. 10 combinations).So you can have 9x109 = 9,000,000,000 combinations.
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.