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.