A Barrel Roll is a maneuver where the aircraft flight path follows what would be the surface of a huge horizontal barrel in the sky. To do this you of course need to roll the aircraft. To roll the Phantom, no matter what maneuver you are attempting, requires that you create differential lift on the wings. (more lift on the up-moving wing than on the down-moving wing). At low angle of Attack (AOA), in any model of F-4, you use the ailerons/spoilers to do this. At higher AOA, the ailerons would induce adverse yaw, and therefore you need to create this differential lift by yawing the aircraft with the rudder (since it's a swept wing aircraft). As AOA increases above 10-12 units, the pilot needs to blend in more and more rudder, and less aileron, until at high AOA (19 units in early non-slatted aircraft like F-4B/C/D), he is using rudder exclusively. In later slatted aircraft (USAF F-4Es, F-4Js, USN F-4J/S, etc.) this effect was somewhat mitigated by the slats (they energized the airflow over the top of the wing), and the pilot could use some aileron throughout the AOA range, (but still less and less as AOA increased), all the way to 25 units AOA.
As to your other question about slat deployment, this happened automagically, based on AOA, at about 12-14 units AOA as I recall. And then would retract when AOA fell below 10-12 units... No pilot input was necessary.
So, in the execution of a Barrel Roll in the F-4, starting at 450 KIAS and low AOA at the entry point, the aircraft would climb and descend 4-5000 feet, and slow to somewhere between 200-250 KIAS and higher AOA at the high point, and then accelerate back to 450 KIAS and low AOA at the completion of the maneuver. As a result, the need for aileron and rudder to effect the roll continuously and gradually changed throughout the maneuver.
NOTE. (to address comment made in the question relating to photograph). The F-4's ailerons did move up slightly when the stick was deflected towards that wing, but at high AOA, the aerodynamic effect was negligible due to the wing blanking the airflow back there. SO the F-4 also had spoilers on the top of the wing that would deflect upwards as well, both to decrease the lift on the wing and cause a roll in that direction, and to add drag on that wing to balance the drag from the down aileron on the opposite wing. The control surface deflected on the upper wing in the photo is the spoiler, caused by the pilot pushing the stick to the left.
The following is from the USAF F-4E flight Manual: