If we were actually planning to play 4231, we would need two more attacking midfielders/wingers, so except if we end up getting Dembele and Coutinho, I don't see that happening.
Out of dembele and coutinho, Dembele is the most likely to come, He'll occupy the right wing, Denis or deulofeu can play as LW if coutinho doesn't also come.
------------Marc------------
semedo-pique-umiti-alba
-------Busi-Pauli------------
Dembele-messi--Coutinho/deulofeu
----------Suarez------------
This is a deadly line-up, Strong offensively and defensively.
Yet i doubt valverde will use 4-2-3-1 on consistent basis, I think he'll stick with 4-3-3 for most of the time, It's by far the formation we're most comfortable with.