For this kind of question, start from the left, and use the greatest possible digit for each position. Thus, in the first position you would have a 9, in the second an 8, etc.
987,653
The greatest 4-digit number with no repeated digits is... 9876
98,764
It is possible to create a 3-digit number, without repeated digits so the probability is 1.
757
987,654
987653
9876543
987654
98765
98,764
98,764