>>63954810
It seems to be a cooling inlet.
Not all kits of the F4 have it but I've seen it on both F4Es and F4Bs.
The tail is behind the engines and there are loads of electronics in the vertical stabiliser, antennas and such.
The modernised ones have their RWR in there too.

>General arrangement of the RF-4B. Source: NATOPS NAVAIR 01-245FDC-1 (1967) flight manual
Source: http://aviation.watergeek.eu/f4b-panel.html