Some dogs at the park are well behaved & social with other dogs, while some are not. Some dogs want to remove the other dog from the planet for whatever reason.
I'm not a fan at all of dog parks & see no value in them but some people honestly do & it works for them.
He's literally a pup at 4 months. Pups are playful, goofballs.
I want to control other dog interactions especially when I have a pup, trusted dogs period. I position myself as the leader & one of my responsibilities is to protect my pup & watch the situations I put him in.
I don't believe your pup was being aggressive. He could however been on the defense because of another dog or just overly excited causing his hackles to rise.