There seems to be two issues here. One regards the crews who are slowing slightly before 4nm to be stable at 1000' AGL. The other regards the crews who slow down miles before they should without advising ATC. The latter crews seem to be the problem. Personally in the London TMA even if I am given no speed control I will advise ATC of any significant change of speed. For the former issue I've worked out some figures on the back of a fag packet. An aircraft starting to decelerate 0.5nm early the aircraft will arrive at the runway 2 secs later will effectively be 100m further back and 20 ft higher. IMVHO this does not seem to be enough to really effect separation.