Why on earth are you using a prong collar on a 4 month old puppy, and why on earth would you let a "trainer" step on the leash connected to the prong and hold it down to apply intense pressure to a sensitive puppy's neck? That sounds abusive, IMO. It's also counterproductive... pain and fear in a puppy's mind is damaging, no matter the genetics in the dog.
4 month old puppies should be getting pure positive reinforcement to ensure they know what is expected of them and what commands mean before they are old enough to endure negative reinforcement.
Find a new trainer, ditch the prong collar until the puppy is older.