I was always led to believe that the T4 used up airframe fatigue life at a greater rate than the T3 due to the more powerful engine and the operation of the aircraft in high energy environments such as TWU and the College of Air Warfare. I'm probably about to be dispossessed of that notion by others far more knowledgeable than me. None-the-less, I do know that the airworthy airframes generally rotated through or, latterly, "gravitated" to Shawbury where they were the treasured mounts of the gentlemen pilots of Marshalls of Cambridge who provided targets for trainee air traffic controllers at the Central Air Traffic Control School. But you probably knew that anyway.
Sadly that all stopped on 5th July 1989. Happy days.