I would go immediately to a qualified behaviorist. You can find a ton of good advice on the forum and in books, but really you need someone who can help you in person. Especially if your dog's future and the the safety of those around him depends on it.
That being said I will now give you advice to do in the meantime.
1. Do NOT punish your dog for growling. Growling is his way of saying "i'm scared." IF you take away that growl, you still have a scared dog, but now you have a scared dog who won't say anything until the person gets closer and closer, and eventually he will be passed his threhold where he thinks he has no choice but to bite.
2. Start a counterconditioning program. Teach your dog that men equals lots and lots of treats. Start at a point where your pup is ok, then have a man enter the room, shovel treats into pups mouth, man leaves, treats go away. REpeat. The only predictor of treats should be the arrival of people, so make sure your hand reaching into your pocket means nothing. Eventually you are looking for your pup to have a "where's my chicken!" expression whenever he sees a man. Once you see this, make it harder, the man comes closer, the man looks at your pup, the man moves about, etc. Work slow enough so that your pup never gets above threshold. Do not try and have men pet your puppy at this point, or even hand your dog a treat. Hopefully this will go quickly since your puppy is only 4months, but still the socialization window has closed and it will be hard. If your puppy is already ok with most of these steps, then maybe just work on having lots of men throw your puppy a treat and walk away. Eventually they can feed from the hand, but not right now.
Try not to put your dog in situations where he's above his threshold. Petsmart may not be a good idea right now depending on how fearful he is, or it could help a lot if he's not that bad. Just make sure that you know what to say to approaching people who likely want to hug and squeeze your little pup.
3. Get the books "Cautious Canine" by Patrica McConnell, and "How to Right a Dog Gone Wrong" by Pamela Dennison.