TextArea size

Hi,

Wants to limit the text area to 4 characters...How Can I do this?
Iam using java script like ,

{
name : "pin",
title : "PIN",
align:"center",
textArea:"isc.TextArea.setCharacterWidth(4)"

},

But it is not effecting , still allowing to enter more than 4 chars..

Can anyone tell me how can I do that please?