I am so sorry. I can't imagine that kind of pain you must all be feeling.
I would be as truthful to your boy as possible, but definitely keep out the ugly details. Just tell him the pony got into an accident and died peacefully. He will probably ask questions, but just be vague. Don't lie, but skip the details. He may ask again when he's older, and there may be a time when he should know what really happened...but at age four, he has a lot to worry about.
However, you should warn him about the dog if he comes back. Whether you tell him that the dog had anything to do with the pony is up to you, but at age four, he should know to stay far away from that dog. I wouldn't let him go out by himself, but he should be aware that the dog is dangerous.
Riders aren't 16 and pregnant. Riders are 16 and arthritic.