The smallest four digit number with four different digits is ______________

Four smallest digits are 0, 1,2 and 3. In order to write the smalest 4-digit number using digits 0, 1, 2 and 3, we put the smallest digit 1 (Except 0) at the place having the highest place value. The largest digit 3 is put at the right most place i.e. at unit's place, the digit 2 is put at the ten's place and the digit 0 is put at the hundred's place. Hence, the required largest number is 1023.

The smallest four digit number with four different digits is 1023 .