I will be honest with you; I’m not really sure what to tell you. Biting is such a broad topic that I hardly suggest any training techniques for it over the internet. Without seeing the dog do it in person, it’s close to impossible to know exactly why she is doing it. People often believe the dog is biting because of this or that, but it could be something totally different. Some dogs bite because they have an infection and are suffering from it; others do it because they have learned that it is an appropriate way of getting their way around.
Maybe you should try contact another behaviourist, one that can come to your house and examine what you and your dog are doing. She is only four months old – this can be fixed if you act immediately. There was another member on this forum a while back that had gone to three different trainers before he was able to see some results. The first two focused on “dominance” and how to break the dog. Well, what that created was an atmosphere where he didn’t even like his dog anymore. He started some positive training techniques along with strict rules and a lot has changed, he sent me an e-mail.
It is very important that you fix this problem now. If she is big now, can you imagine how big she will be when she is full grown? That is probably why the dog trainer advised you to put her in a shelter, if this problem isn’t fixed now it will be very dangerous later on.
All I can say is to not give up. But I don’t believe you should tolerate any kind of biting whatsoever. Having a family dog should be fun and harmonious, and I believe that is why you have gotten a second dog.
You could always try some positive training techniques with her, to see how your relationship develops with her. Clicker training is pretty easy and fun, all you need is some treats and a little bit of patience. As a beginner you should take a look at the Luring Part 2 lesson which I simply touch my dog, click and then feed. This usually creates a positive association with you being close and touching the dog. I’ve done this with my neighbour’s dog, she was a timid dog where you couldn’t even walk behind her or touch her side without her backing away. With this technique, in two weeks, I was able to step over her and touch her anywhere I wanted.
Eventually you could do this with the muzzle as well. It’s obvious that she hates her muzzle, but I’m curious as to how it was introduced. If it was forced on her using your strength, then she probably developed a very negative association with it. This could be done by putting the muzzle on the floor and letting her see it, (click & treat), then sniff it (click & treat) and then you could move on to getting the muzzle around her face, (click & treat). This is the same thing as the previous exercise; it creates a positive association with the object by using food.
I hope this helps a little.
Jean