Because the original design was intended for a somewhat lighter aircraft, the F4H (F-4) Phantom II wing underwent some subtle changes in development to provide more lift to go with the increase in gross weight. The first of these were the addition of the inboard leading edge flap and boundary layer control to the sixth F4H. (See HERE and HERE.) Note that I've added information from Craig Kaston about the earliest changes to the stabilator. Also see: http://tailhooktopics.blogspot.com/2012/11/f-4-stabilator.html
As on all airplanes that don't have the highest level of pitch stability augmentation, the F4H stabilator created down load in most flight conditions. The original stabilator had a symmetrical airfoil.
The Phantom's gross weight continued to increase over time with upgrades, fixes, additional equipment, etc. It was clear that with the next round of improvements planned for the F-4J, another lift increase would be required. Step one was obvious, drooping the ailerons when the flaps were lowered: 16.5 degrees was determined to be adequate from wind tunnel test.
As expected, the droop resulted in more nose-down moment with the flaps extended, too much as it turned out. The problem was most obvious with the increase in nose wheel liftoff speed and the inadequate pitch control power after catapult launch and during bolters. Additional wind tunnel tests established that elimination of the inboard leading edge flap (added during development as noted above for increased lift) reduced the nose-down moment at nose-wheel liftoff speeds and turned out not to affect lift at approach speeds.
Although eliminating the inboard leading edge flap was beneficial, it was inadequate. Further wind tunnel tests indicated that the stabilator was stalled at nose wheel liftoff speed. As McDonnell engineer Bill Weber remembers it, "We tried a matrix of planform and area changes to correct the problem and finally determined that adding a fixed leading slat would delay the stall and could fix the problem. (Note: These wind tunnel tests were conducted without a simulated jet exhaust - later tests with jet effects indicated that the slat did not delay the stall and the improved stabilator power was probably due to an increase in effective area.)"
Even before the wind tunnel tests of the slotted stabilizer were accomplished, McDonnell program management decided to fabricate a slotted stabilator by adding a fixed slat to the leading edge of the existing one and flight test it. Bill Weber again: "As a consequence flight tests of the slotted stabilator took place very shortly after we had wind tunnel results. In any event the flight tests demonstrated that the problem had been fixed. We used the same configuration changes to fix a similar problem on the F- 4E which didn't have aileron droop but had a more forward C.G. due to the installation of the gun."
Although created for the F-4J, which first flew in June 1966, the drooped aileron and associated changes (the slotted stabilator and the elimination of the inboard leading edge flap) were of benefit to the F-4B as well. The package of changes was incorporated with Block 26 production, the first of which was BuNo 152995 that first flew in March 1966, and retrofitted to most of the surviving earlier F-4Bs over time.
Retrofit involved the development, qualification, and approval of a different package of drawings and other documents for use by the Naval Air Rework Facilities before crash-damaged and overhaul-due F-4Bs would get these changes. I've read that "by late 1971 or early 1972 it would have been rare to see a F-4B without these modifications, except perhaps in RAGs or Marine reserve units." For sure the slotted stabilator was present on F-4Ns coming off the Bee Line at North Island. The first F-4N flew in June 1972.
This is an F-4N, which is an upgraded F-4B. Note the drooped ailerons and fixed inboard leading edge along with other detail changes to the original B configuration.