Write a number that has four digits with the same number in all places such as 4444 circle the digit with the greatest value underline the digit with the least value

For a four digit number we have four places.

Ones, tens, hundreds & thousands.

And all the four places should be same

So we have

1111, 2222, 3333, 4444, 5555, 6666, 7777, 8888, 9999

Of all these the largest value at thousands place is 9.

So 9999 is the largest number

Also 1 is the smallest value and so 1111 is the smallest.

Hence

9999 is the largest & 1111 is the smallest.