At 4mo he is still socialising and learning, the fact they turned around and started running will definitely make his prey drive kick in. Start obedience training, if you can. Join a local group where he trains among other dogs and make it very clear to him from the start that 'training is training' it is not a time to meet and greet and play. Obedience training can help build a better bond between you two and apart from mastering the recall, the other essential thing to master is focus. Find something of really high value to him (food, ball, your high pitch pleased voice etc) and use it to reward him for each focus on command. Give him the command to 'look' or 'watch' and the second he looks at you, reward him. Training a dog to focus on you will help you in the situation you were in, he will learn to ignore outside stimuli and look at you. Be consistent and find that high value reward - you will need it with a high drive GSD.
Good luck with your finger - sounds very paiinful
if your GSD is eating and eating and eating and losing weight - please consider testing for EPI.
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