There are twelve possible solutions using the rule you stated.
98765432
It is 98765.
9876543
987654
987654321
There are 840 4-digit combinations without repeating any digit in the combinations.
98765432
123,456,789
9876543
It is 98765.
9876543
987
987654
987654321
10234567
I believe it is: 987,654
9876543