The 1st digit is 1 out of 10 numbers
The 2nd digit is 1 out of 9 numbers
The 3rd digit is 1 out of 8 numbers
The 4th digit is 1 out of 1 numbers.
Total combinations are 10 x 9 x 8 x 7 = 5040
Since the combination 7654 is one out of the total the probability would be 1/5040
