The fourth qualification is called Shatka Sampatti in Sanskrit. Shatka means six-fold. There are 6 sub-qualifications within this particular qualification, and all except one (Shraddha), is related to some sort of discipline.
So to make it easy to remember, we will call the main qualification as “Discipline”. Also, since the previous three qualifications all start with the letter “D”, we can call the four qualifications as the 4 D’s: Discrimination, Dispassion, Desire for freedom and Discipline.
The six sub-qualifications of Discipline are: