How to limit NSTextField text length and keep it always upper case?

Need to have an NSTextField with a text limit of 4 characters maximum and show always in upper case but can't figure out a good way of achieving that. I've tried to do it through a binding with a validation method but the validation only gets called when the control loses first responder and that's no good.

Temporarly I made it work by observing the notification NSControlTextDidChangeNotification on the text field and having it call the method:

- (void)textDidChange:(NSNotification*)notification {
  NSTextField* textField = [notification object];
  NSString* value = [textField stringValue];
  if ([value length] > 4) {
    [textField setStringValue:[[value uppercaseString] substringWithRange:NSMakeRange(0, 4)]];
  } else {
    [textField setStringValue:[value uppercaseString]];
  }
}

But this surely isn't the best way of doing it. Any better suggestion?