The HK-Drone, also known as T-4 Aerial is designed by Cyberdyne Research Systems to be an unmanned aerial weapons systems which the organization was attempting to market to various military powers prior to 2004. As thousands were produced, when Skynet came on line, it immediately had access to a small army of sophisticated tactical air craft. Post atomic holocaust humans would become quite familiar with them.
The HK-Drone had the least sophisticated CPU of all Non-Humanoid Hunter Killer units. Its artificial intelligence and decision making abilities, while fast, were limited to insect like reasoning. Its inferior sensory array did not complement this lack of brains. For instance: an HK-Drone which has fired a missile will then chase the debris of the explosion it created as a living target. 
It has difficulty discerning a non living object from a living one. To it, anything warm and moving is a target.
Armed with conventional arms (machine guns) and low-yield missiles. Lacking the fusion reactor power source of later HK-Aerial's, the drone had no plasma phase weaponry. This would be introduced by Skynet in the model's successor, the FK.
Speed and Maneuverability
The original HK-Drone utilized the same VTOL design as the later large HK-Aerial and compact FK, yet more primitive. It had three swivel mounted turbo fans positioned around its body giving it great maneuverability and speed. It had more limited range than any Aerial HK due to its lack of a fusion reactor power.
The HK-Drone had lightweight titanium armor somewhat resistant to conventional fire power, but sustained fire from conventional small arms will eventually destroy it. It has no resistance to plasma phase weaponry or ballistic projectiles.
The HK-Drone did have the advantage of having little surface area to target compared to most aircraft, as it was only about 2 meters in diameter from wing tip to wing tip and slightly less from nose to tail.
- There is an alternate unit referred to as an HK-Drone appears in the game The Terminator: Future Shock. This unit is a small, nimble aerial vehicle used to scout areas for Resistance soldiers and alert larger HKs. It is armed with a low power laser weapon which it only uses at close range. Like the HK-Drone in Terminator 3: Rise of the Machines, this unit also has weak armor.
- Video games