Is differential horizontal tail deflection used to roll the aircraft around the longitudinal axis along with the ailerons / spoilers and a bit of rudder?
No, the two sides of the Stabilator are connected to one another and move in unison. The following is from the USAF F-4E flight manual, page 1-21.
(Charles Bretana's answer provides direct evidence. This answer adds historical context.)
Other fighters use differential elevator to cause roll, F/A-18, F-100, F-15, but not F-4:
F-4 pilots were trained to use the rudder to roll the aircraft at higher AOA. Instructors were trained to emphasize this flight characteristic, and, whenever at high AOA, to keep our knees in the back seat right up against the control stick to inhibit new pilots from introducing any aileron.
R. Kogler's Automatic limiters in air force aircraft flight control systems confirms that
...pilots originally trained in aircraft like the F-4 were taught to roll at high AOA with rudder commands rather than lateral stick commands to avoid departures [from controlled flight] due to adverse yaw from ailerons and differential tails.