Those are Mk 4 Mod 0 pods, 3 per aircraft. One centerline and two outboard on the wings devised by VMFA-122 armament team at Da Nang. Developed by the US Navy, this pod is fitted with the Mk 11 Mod 5 20 mm cannon, along with 750 rounds of ammunition. The gun had a reputation for jamming which posed a recoil problem to the aircraft if just one of the wing guns jammed. The high explosive rounds could also tumble and explode in front of the aircraft which then flew into the shrapnel created. It was used on a variety of US Navy and Marine Corps aircraft including the A-4 Skyhawk, F-4 Phantom II, A-7 Corsair II, and OV-10 Bronco. Without a gunsight they were useless in a air-to-air engagements and inaccurate in strafing runs unless frequently boresighted plus the drag induced by the pods reduced performance.
The F4-E finally received an internal 20mm Vulcan cannon