please not source dear god
Mind me asking why you are against the Source engine? Is it because it is made by a Valve? Do you hate Valve, you would be the first that I know about. Is it the age of the engine? Valve has kept on updating it and make most of their games on it and they still look good.
Anyway, chances are that Fallout 4 will be using the Creation engine that Bethesda has created.
But if I had a choice in which engine it would use, I would chose the Rage engine that is made by Rockstar. The engine has already shown that it can do big and detailed maps. The only problem, is that there has yet to be a RPG based on the engine.