This from Alessandro Zauli’s “Soccer, modern tactics” in which he interviews a coach called Maurizio Viscidi about the 4231 formation.
The key issue here is that with four attacking players you can be outnumbered in defence. We did okay at avoiding this because Dembele in particular worked very hard to get back, but the principles still apply. The diagram on the bottom right shows the problem: ideally you want the full-backs to be closer to the wide players, but if they do this then you have uncomfortable situations in the middle where the centre-forwards are effectively one-on-one with the centre-backs. So the full-backs have to tuck in to help, meaning there’s lots of space out wide, which ideally your wide midfielders would help out with. Except they’re in this very fluid three up the pitch so it’s hard for them to do this.
Manchester United exploited this ruthlessly, pulling our defenders all over the place.