Everything is explained on the title but i want to tell u the reason why. This is simple when the season is over (in real football) the players have vacation minimum 2 mounts. So in this game 4 days is like one mount that means even if in this game the players have 1 mount of rest they will be fully fit and ready to play.
Here is that calculation:

1 Day = 40% Condition
2 Days = 80% Condition
3 Days = 100% (Full) Condition
4 Days = 100% (Full) Condition