title: Replace characters in NSString
body: <p>I am trying to replace all characters except last 4 in a String with *'s.<br>
  In objective-c there is a method in NSString class <code>replaceStringWithCharactersInRange: withString:</code> where I would give it range <code>(0,[string length]-4)</code> ) with string <code>@"*"</code>. This is what it does: 123456789ABCD is modified to *ABCD while I am looking to make ********ABCD. 
  I understand that it replaced range I specified with string object. How to accomplish this ?</p>