string doesn't change

Hardware Arduino Due
7

I'm using an ARDUINO DUE (IDE version 1.6.3)!
The following code

void setup() {
  Serial.begin(115200);
}
void loop() {
  printOnDisplay("This is a test");
  while(1);
}
void printOnDisplay(char *string)
{
  string[4] = 0;
  Serial.println(string);
  if (string[4]) Serial.println("String hasn't changed!");
  else Serial.println("String has changed!");
}

leads to the output:
"This is a test"
"String hasn't changed!"