Find the greatest 4 digits number which is a perfect square.

We know that
The greatest 4-digit number = 9999
Now we have found the perfect square by using the long division method of the given number,

Thus, the remainder = 198
When we subtract 198 from 9999 we obtain a perfect square.
9999 − 198 = 9801 
Here,
$\sqrt{9801}=99$. 
Hence, the greatest 4 digits number which is a perfect square = 9801.