title: NSString append WhiteSpace for every 4 Character
body: <p>I have a string like @"1234123412341234", i need to append space between every 4 chars like.</p>
  
  <pre><code>@"1234 1234 1234 1234"
  </code></pre>
  
  <p>i.e, I need a <code>NSString</code> like Visa Card Type. I have tried like this but i didn't get my result. </p>
  
  <pre><code>-(void)resetCardNumberAsVisa:(NSString*)aNumber
  {
    NSMutableString *s = [aNumber mutableCopy];
  
    for(int p=0; p&lt;[s length]; p++)
    {
      if(p%4==0)
      {
          [s insertString:@" " atIndex:p];
      }
    }
    NSLog(@"%@",s);
  }
  </code></pre>