I have served on the boards of three off-leash dog parks and created one. I tend to have strong ideas about this.
Socializing a dog? Yes, the dog park is a great place for that. Bringing an untrained dog to a dog park? Absolutely not.
Every dog park that I have ever seen has rules that say the dog must be under voice command of the owner at all times. That means you dog must come if you call it. It must stop what it is doing if you tell it to.
Petco has a series of free seminars you should look at. Also, their puppy training classes are not that expensive. You can also get a lot of information and ideas and tips on training basic commands on the Web.
When you are at the dog park, have your dog in sight and in range of your voice at all times. (Some people sit down and ignore what their dog is doing. Not good.) Especially with a four-month-old puppy. That is usually the minimum age you can take a dog to the dog park. Dog status-wise, your dog will be low man in the pack hierarchy. You need to watch carefully to make sure he is not hurt by other dogs. Even in play, a dog can be bowled over, for example, and damage ligaments.
It is your responsibility to make sure your dog is not annoying another person or dog at the park. It is not OK to let your 70 lb. dog jump on someone and then say “Sorry, he’s just a puppy.” Teach him not to jump on people. Humping another dog (a common thing for young dogs just coming into their sexuality) is not acceptable behavior.
Remember, you are legally liable for anything your dog does at the park. So, you want to make sure the dog has basic training.
Otherwise, have a great time! It will be a blast for both of you.