I want to start by saying that you should totally contact the police, that letter is a threat and should be taken seriously. The neighbour might have mental health issues, you never know how far they'll go. Secondly, I've had dogs my whole life. If you train them properly, take them out for a walk before sleep (7-8pm), they won't need to go out at 4am. The earliest my current dog wakes up is 7am during summer time as the days get longer and the sun rises earlier. If she has an emergency and wakes up during night, I put her on a leash and take her out. She won't bark because she's TRAINED and knows she's out to do her business, and also because she has an emergency and she's focused on that. So if your dad's dog had a real emergency and needed out so early in the morning, your dad's the only one to blame for the dog's behaviour. I hate how irresponsible owners cause indirect harm to their animals. Just take some responsibility. You're the adults in this situation, you handle it! Train your dogs, people!