All of the whole numbers from 1,000 to 9,999.
That's 9,000 positive ones. I guess you could also count the 9,000 negative ones.
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Six.
10000 ; This may look to be wrong but you can have 0001 to 9999 which gives you 9999 different combinations of 4 digits, plus another one for 0000.
4: 1999,9199,9919,9991
There can be 103 = 1000 codes.
3024