From a boffin - Main reasons for the displaced threshold.
1. Before QR was built the capacity on 04 arrivals was less than 22 because the longer distance from the original threshold to
PR and it's 30 deg angle design increased runway occupancy time. Also the touchdown zone using the original 04 threshold was uphill which had the added effect of slowing aircraft quicker and taking longer to get to
PR to vacate the runway. Now that 04 is displaced and QR has been built to a lesser 25 deg angle design this means that the touch down zone is on the crest of the hill and the exit points are a similar distance and design to those on runway 22 so arrival capacity is similar for both runway directions.
2. At the same time that QR was built for 04 arrivals the runway 22 intersection take-off point at Q was also built to assist with capacity for 22 departures and this can also serve as a 04 exit point for "heavies" or others which miss the QR exit. This meant that a 3rd runway 04 exit doesn't need to be built to maintain runway capacity when accommodating "heavies" in the long term future, so keeping costs down
Other benefits of displacing the 04 threshold:
5 x bar approach on 04
Northside movements can get get closer to the 04 Cat I holding point at G1