Many of us have been through that face. Trust me. Please do not make a judgement on how your dog is going to turn up as an adult, based on behavior of 4 month old pup. My female GSD had and still has high dog aggression, but she is turning out be a great gsd winning shows, and very well mannered companion dog. My male pup (six moths) will lunge at cows and always looking for a fight.
Best thing is to socialize them with people and other animals which you are already doing. Just when you see a dog or live stock approaching your pup, go close to your pup say "Easy Easy". If you see he is getting tensed (hackles up) change the direction of your walk and take him away (increase the distance between your pup and the other animal).
Every time your pup tries to lunge, say a firm "No" and the moment you see that he is not lunging but watching the other animals closely, immediately praise him. It will take some time. But you will be fine. (After all you have GSD, not a Lab, Ha Ha).
Last edited by TEZPUR1976; 09-13-2016 at 01:08 AM.