In order to make the number as big as possible, you would want the greatest value digits in the greatest place value. Thus the first number would have to be 9. Once 9 has been used, the second number would have to be 8. Then the third number would have to be 7 and so on. Therefore, the greatest 9 digit number with 9 different digits is 987,654,321.
9,999,876 is the greatest seven-digit number using four different digits.
The greatest 4-digit number with no repeated digits is... 9876
9876 and 1234
That would be 99987.