{6,4,8,0}

Write the largest possible first digit, 8

Of the remaining digits {6,4,0} write the largest possible second digit, 6

Of the remaining digits {4,0} write the largest possible third digit, 4

The last digit must be 0

So the largest possible number is 8640

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{6,4,8,0}

Write the smallest possible first digit, it cannot be 0 because 0 is not 
really a first digit. So you must use 4 for the first digit.

Of the remaining digits {6,8,0} write the smallest possible second digit, 0

Of the remaining digits {6,8} write the smallest possible third digit, 6

The last digit must be 8

So the smallest possible number is 4068.

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Subtract them 

8640-4068 = 4572

Edwin