Back 4 won't work long term unless you upgrade the fullbacks and center midfield - 4 new players IMO.
Aye, centre midfielders put unneccesary pressure on the defence too often. You need a Cabaye-Tiote-esque duo and we currently have nobody near their level.
Fwiw, I think we should keep working with 4 at the back, at least more regularly. The front 4 showed great potential, and Schar was excellent despite constantly hearing he can't play in a 2.