Dear Leena,
The reason is that,
Java uses UNICODE for representing charachters internally, but uses UTF for IO.
UTF means UCS Transformation format
and UCS menas Universal charachter system.
UTF uses 1 or 2 or 3 bytes to represent charachter in IO
and all ASCII char's requires 1 byte only.
Thats why answer is 4 bytes.
Sujit