Write the greatest 4-digit number having distinct digits.

The digits available are 0, 1, 2, 3, 4, 5, 6, 7, 8 and 9.

For the greatest 4-digit number with distinct digits, arrange the largest digits in descending order:

Thousands place: 9 (largest)

Hundreds place: 8 (next largest)

Tens place: 7 (next largest)

Ones place: 6 (next largest)

Hence, the greatest 4-digit number having distinct digits is 9,876.