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Assuming the digits cannot be repeated, there are 7 combinations with 1 digit, 21 combinations with 2 digits, 35 combinations with 3 digits, 35 combinations with 4 digits, 21 combinations with 5 digits, 7 combinations with 6 digits and 1 combinations with 7 digits. That makes a total of 2^7 - 1 = 127: too many for me to list. If digits can be repeated, there are infinitely many combinations.
10,000.
There are infinite combinations that can make 3879
45
If you have 4 positions, each of which can hold any of the ten digits, you have 10 to the power 4 combinations. If you can have only 4 different digits, you have 4 to the power 4 different combinations.