Write down the greatest number and the smallest number of 4 digits that can be formed by the digits 7, 5, 0 and 4 using each digit only once.

The given digits are 7, 5, 0 and 4, and the repetition of digits is not allowed.

For the greatest 4-digit number, the digit at the thousand's place has to be the greatest, which is 7; followed by the next greatest digit 5 at the hundred's place, then 4 at the ten's place and 0 at the unit's place.

The greatest 4-digit number = 7,540.

For the smallest 4-digit number, we have to follow just the reverse. The digit at the thousand's place has to be the smallest non-zero digit, which is 4 (since 0 cannot be at the thousand's place); followed by the next smallest digit 0 at the hundred's place, then 5 at the ten's place and 7 at the unit's place.

The smallest 4-digit number = 4,057.

Hence, the greatest 4-digit number is 7,540 and the smallest 4-digit number is 4,057.