At only 4 months old he's too young to be stuck into a group of strange dogs, especially adult dogs.
See if you can find a puppy kindergarten classe near you, or a puppy playdate class. I was able to find a training center that offered both, I got my puppy when he was 4 months old and took him to those classes once a week until he was 6 months old. He was nervous the first day too, and wouldn't play, but the second time he went, he had a blast, and it continued that way. It was good for him because it gave him confidence around other dogs, and it taught him social skills for interacting with other dogs.
At the dog park you don't know who those dogs are or how aggressive they are, they could badly frighten a puppy, or even injure him, you just don't know. I didn't know the puppies at play date either, but at least they were puppies, and so much less likely to aggress.
What you are seeing is not a Doberman thing, it's a puppy thing.