8,754,321
8643
531
999
The biggest 4 digit number that can be made from using the digits of 850236, using a digit at most once, is 8653.
9531
-4790281
3201
9876543210
Assuming each digit only once: 65432 (and no further mathematical operations).
The biggest number that can be made by arranging the digits is 951. However, 9^51 is a little under 5 trillion trillion trillion trillion, much larger than 951.
The smallest number that someone can get using the 91764 digits is 14679. The secret is to arrange the digits from the least number to their greatest number.
9876543210 as a string. However, if you allow other ways of combining digits, the answer becomes impossible - since there is no limit to the ways in which such operations might be defined.