OMG how terrible
I hope you find him. He is in my prayers.
I would definitly go and check out the dog that is 4hours away. If you don't you'll always be wondering if it was him or not. It's definitly worth a try.
Also, don't blame yourself. You didn't know this would happen, and your doing everything you can. Ace is very lucky to have a mommy who loves him so much.