Drop in stamina does not depend on minutes played (it did 2 seasons ago, but this has been changed).
It does depend on the current level of stamina (players high in stamina drop more than those low in stamina). I'm not sure how high you can crank stamina up to. I know you can get proficient, but I'm not sure about prominent.
What is stamina of the players now?
4 is a lot of drops, but I think you actually get a small decrease each week, and then every so often it will be visible as a plop(?).