The biggest 4 digit number that can be made from using the digits of 850236, using a digit at most once, is 8653.
Assuming each digit only once: 65432 (and no further mathematical operations).
The biggest number that you can create using 1234 is 4321.
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.
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.