how to find the greatest possible number that can be rounded off
there is no greatest possible number that can be rounded.
However, if you want to round to a particular place value, then the greatest possible number depends on how many significant digits you have, and how many you want to retain.
If dealing with integers, then the greatest possible integer that can be rounded to 4000 is 4499.
If you are allowing 6 significant digits, then it would be 4499.99
and so on.