If the question is about 4 things out of 4, the answer is, obviously, 1.
If the question is about 4 things out of n, then the answer is nC4 = n!/[(n-4)!*4!], whose value will obviously depend on n.
4
216
Assuming that the only permitted operation is addition, then there are 4 combinations.
There are 210 4 digit combinations and 5040 different 4 digit codes.
There are 840 4-digit combinations without repeating any digit in the combinations.
4
There are 5,040 combinations.
There are 120 permutations and 5 combinations.
216
Assuming that the only permitted operation is addition, then there are 4 combinations.
120 combinations using each digit once per combination. There are 625 combinations if you can repeat the digits.
4582
20
9000
There are 210 4 digit combinations and 5040 different 4 digit codes.
There are 840 4-digit combinations without repeating any digit in the combinations.
10,000