the prong collar will work better if it's fitted properly. you need to pop out as many links as it takes for it to stay snug at the top of the neck. when the dog walks ahead and runs out of lead say heel while giving a slight tug on the leash. ever so slight that's it's only 4 months old.
i try to steer clear of training advice not being a trainer and only have a year of dog experience and 2 GSDs but teaching to walk good is a pretty basic thing.