Sorry, I don't think spinning hook kick is detrimentally important to Hapkido as an overall style. I'm really looking for very specific defense techniques, done in a specific order, much like a form. I originally didn't give much detail because I wanted to leave it a little vague, but I suppose that was a mistake as I apparently left the question too vague.
Like I said, no one could ever honestly break down the entire art of Hapkido into only four different techniques. I'm yet to come up with a total of four techniques that would qualify as a hypothetical break down, but the two that I have considered are as follows:
1 -- (against an outside/straight wrist grab to the right hand) Create an open live hand with the right hand and bring the hand to your center with your right palm facing up, and at the same time grab your opponent's left wrist with your left palm facing down and your left thumb pointing to your right. At the same time step toward your opponent with your right foot into a horse stance, and at the same time pull your right hand toward your left side to release your opponent's grip. With your left hand still grabbing your opponent's wrist, pull your opponent toward you and execute a right outside elbow strike to your opponent's ribs. Maintaining your grip on your opponent's wrist with your left hand, place the blade of your right hand on your opponent's left elbow and execute a standing arm bar.
2 -- (against a double front/collar/throat grab) With your left hand grab the middle of your opponent's arm, with your left palm facing down and your left thumb pointing toward your opponent, while at the same time stepping with your right foot to your opponent's right side into a horse stance, while at the same time placing your right hand on your opponents lower back. By only turning parts of your body, but without moving your feet, execute a right hip throw and drop your opponent to the ground, while maintaining your grip with your left hand on your opponents arm. Slide your left hand upward to your opponents wrist and create a grip on the wrist while at the same shifting into a left front stance and placing your opponent's elbow on the side of your knee. With your right hand prepared in a closed fist at your right side, shift downward into a left kneeling stance while execute a low right hand punch to your opponent's head, and at the same time pulling your opponent's right arm to the left of your body, in a motion that is meant to break or fracture the arm. If necessary, follow up with a right hand inward palm strike to the back of your left hand while pushing against your opponent's elbow with your left knee.
Done at normal speed, each technique really only takes a few seconds, and maybe less for the first one. But each technique includes a fair variation of individual techniques. Between the two techniques are covered the ideas of centering, striking vulnerable areas, joint manipulation, and a take down. By no means would mastery of these two techniques equate to a mastery of Hapkido, but that is why this is a hypothetical. My only reason, really, for asking this question, is out of sheer curiosity. Some of you may want to include much longer techniques, in order to cover more concepts of Hapkido. Some may want much shorter techniques in order to keep it simplified. But again, this is only a hypothetical. Thanks.