The smallest 4-digit number is 1000.
The greatest 4-digit number is 9999.
We know that,
The square of 100 is 10000 or 1 more noteworthy than the biggest 4-digit number.
So, the closest square root of the largest perfect square is 99. So. The square of 99 is 9801
Hence, The greatest 4-digit number is 9999, and it is the closest square of 99.