Early in the F4H Phantom design, it was determined that more lift was needed to achieve the approach air speed required. The solution was the addition of an inboard leading edge flap and boundary layer control over the other leading edge flaps and the trailing edge flaps. This required postponing carrier trials from the 4th airplane to the 6th, since low rate production was already under way by the time the redesign was released.
Even more lift was required by the time the F-4J was ordered, which was accomplished by drooping the ailerons along with the flaps. The resulting increase in nose-down pitching moment proved too great for the existing stabilator. Locking up the inboard leading edge flap to increase stabilator effectiveness might have been enough but it was decided to also add a fixed slat to the leading edge of the elevator. The package of changes was made to F-4Bs then in production and retrofitted to surviving Bs.