The minimum trading days is simply the least amount of days within which you can complete the challenge.
So, if your prop firms minimum trading days is 4 and you complete the challenge in 3 days, you have breached the rules. You must complete the challenge in 4 days or more.
Minimum trading days was put in place by most prop firms to prevent you from completing the challenge in one day.
They want to be able to eliminate the chances that you just got lucky on one day and they also want to be able to study how you manage risk on different days.
