Assuming each digit only once: 65432 (and no further mathematical operations).
999
Standard Form
If you must use all digits precisely once then the answer is: 8642, 8624, 8462
You get the largest number if you sort the digits, from largest to smallest.
99
8,754,321
8643
531
999
9531
The biggest 4 digit number that can be made from using the digits of 850236, using a digit at most once, is 8653.
-4790281
3201
9876543210
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 you can create using 1234 is 4321.
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.