Another reason I have been told for the moves between T1 and T4 at LHR, from a BA manager...

T4 has greater noise restrictions owing to its location nearer the perimeter of the airport, with the other three terminals in the middle of the airfield. As many long-hauls arrive early in the morning when noise restrictions are at their strictist, aircraft had to be towed to T4 at times. T1 does not suffer this problem obviously.

Additionally, T4 was tight on space, with so many flights coming in early in the morning. T1 being (more) for short-hauls did not have the capacity issues early in the morning. So it logical to move some long-hauls over to the ostensibly empty stands at T1, both to make room and avoid noise restrictions.

This is what I have been told. Please don't kill the messenger.