Six of them.
24680
There are twelve possible solutions using the rule you stated.
96532
9876543210, without using any mathemeatical operations.
The answer is 3*3*2*1 = 18. If the leading digit is 0 it is not a 4-digit number.
24680
There are twelve possible solutions using the rule you stated.
96532
9876543210
9876543210, without using any mathemeatical operations.
93,876
The answer is 3*3*2*1 = 18. If the leading digit is 0 it is not a 4-digit number.
I think the answer is: 976542
648
There are 840 4-digit combinations without repeating any digit in the combinations.
18, if leading 0s are disallowed.
24