For 25 seasons I realised that when you often change your formations,the performance of your players is not too good.And I realised another thing.I like 2-3-4 formations that I use regularly.Some seasons I have success with one formation and I failed with another.Next season is vice versa when I bought some new players in the same positions.
Some players who are DML.ML are trained more at defensive and physical skills,not so much at attacking.ML/AML is vice versa.If you have DML,ML and you like to use red arrows on ML,train him at attacking and physical,and not so much at defensive.
Not so many people realised that primary/secondary positions has matter.There is no matter about the order "DC/DMC" or "DMC/DC".I've just seen difference of performance of some of my players.I have scout player who is DC/DMC.When he is defender he always plays with 7-8-9 rating.When he is DMC he always playes with 6,max 7 rate.
The fourth doesn't matter.