Defensive Electronic Countermeasures (DECM) against Surface-to-Air Missiles (SAMs) was a given for large Navy attack airplanes but not initially for the A-4 Skyhawk because of the size and weight of the electronics required. Losses over North Vietnam dictated its addition.
The exterior change was the addition of five antennas, three forward and two aft. One forward or one aft detected the radar pulse of four different kinds of fire-control radar. The ALQ-51 electronics then automatically transmitted a pulse from the other two forward antennas or the other one aft that misled the radar operator or confused him as to the location of the airplane, preventing guidance of the SAM to it. For an instructive video on the process, click HERE.
The antenna resembled a snow cone:
The forward transmitting antennas were located on either side of the nose landing gear actuator well. Two were probably required because there was no place to mount one aft of that or if there was, the airframe or stores might block the signal from it.
The antennas were exposed and therefore sometimes broken off. In that event or if they were not required, they would be replaced by a half-round black rubber ball as in the mount labeled 9 here:
A subsequent addition to the Skyhawk's ECM suite provided the pilot with a warning that a fire-control radar was tracking him to improve his chances of spotting a SAM fired at him and evading it in case it wasn't being spoofed. This was the APR-25. It utilized four antennas about the size and shape of small drink coasters, two facing forward and two aft. They were angled outward so the APR-25 electronics could analyze the strength of the pulse being received at each antenna, determine the direction it was coming from, and display that to the pilot on a scope in the cockpit.
The two aft antennas were mounted one on each side of the sugar scoop.
The two forward APR-25 antennas were colocated with the forward facing ALQ-51 receiving antenna in a larger fairing.
Thanks to Dave Dollarhide, Bill Egen, Carlton Floyd, and Jim Winchester for their help with this topic; any errors in the above were made by me. Comments, corrections, and additional information are welcome.