Further to what bowly had to say:
Correct, the underground does not go all the way to T4 at the moment (due to work being done on the links for the new T5) but I think you can get off at Hatton Cross and take a very short bus ride from there to T4. Check with the ticket office. I think it may be easier to do this than getting off at T/1/2/3 and trying to get to T4 from there.