MrHorgy,
Believe the reason for the displaced threshold on RW04 was effected as part of the Stansted Gen. 2 development proposals.....the reasoning as part of this was that after analysing the fleet mix using the airport, it would allow the noise contour of aircraft using the aerodrome to be moved closer to the aerodrome boundary. I have seen some charts out there with the various decibel restrictions and associated wind corrections etc., can try and find them if it's of interest, but think that is why you have a 300m displacement.
F/o