If for some reason this affair impacts his ability to get another NFL HC gig, then yeah, it was an abject disaster. Unless he really does not want to be an NFL HC anymore.
I think he will get another NFL HC job someday, but we'll see.
But again, no one is arguing about the money. I'm not sure why you're stuck on that. And it's not just $4 million incrementally. He was going to earn $2.5-3 million for coaching the Bills in 2015. So he's really getting $1-1.5 million to walk away. And now he's getting a fraction of his old salary to be an OL coach.
He might very well end up in net-negative situation 3 years from now if he doesn't land another NFL HC gig. Even if he was fired after 2015 by Buffalo they still would have had to pay him for at least 1 of his 2 remaining years on his contract. So at that point he'd have made ~$6 million over 2015-16 by coaching/getting a termination settlement. Now there's a chance he'll be at less than $6 million if he's in an assistant role in 2016.
I don't really care about the money aspect, but it could easily turn out that he lost money when it's all said and done.