Think using the positional changes as proof of adaptability is a little disingenuous. He was a poor central midfield, a passable stop gap at right back and then found a home as a sweeper. I think it's more a case that he's finally found a role that suits rather turn his hand to multiple roles.
I'd worry about him in a back four because he's more likely to be exposed to 1 on 1 defending and I think he's often been seen to struggle with that. He's ok when forwards are facing away from goal, particularly since Lage joined there's been more emphasis to get tighter sooner and force them back the way they came, and he covers space well to pick off passes on behind. He's poor when someone is running at him though, lack of pace and often seems to get himself in a tangle as soon as someone does anything other than runs in a straight line. We've seen multiple time how physical forwards can beast him when isolate 1v1 too.
I think he'd be ok in a four if you were playing like Burnley, sitting deep and rarely being exposed but then any team with a big physical forward would probably fancy their chances against him aerially in that situation.
He's not the only centre half to have reservations about in a four though. Boly lacks discipline for me, goes chasing the ball too often and that will lead to more gaps to be exposed without a Coady to cover behind. Kilman we just haven't seen play that role, we've seen him grow and become more physical but again it's going to increase the burden taking away a defender from alongside him so maybe he'd struggle a bit, positionally who knows what would happen with him but he's the least risky one to try for me at this point.